Confirming that I’ve experienced this bug as well. What triggered it was alt-tabbing to look at a browser window, then back into the game. Once the diablo window regained focus, none of my abilities were working (despite the action bar lighting up as if the buttons were being pressed).
Rejoining a new lobby fixed the problem and I’ve since not been able to replicate it reliably.
i was in nightmare dungeon, and after use of townportal to sell items and go back into dungeon caused that my skills, teleport, etc. blocked. there was no alt-tab. however this issue was already known by me. I experienced that bug 3 times previously (mostly cellar, helltide (openworld, sidequest-dungeon). And leave game and log in back helps here, but in nightmare dungeon it resets the dungeon (if you are not in a group? so what happens if you are groupleader and you got this bug and you are in nightmare dungeon.)
That happend me a lot.
But one time was a bit different, and maybe that can help debugging:
i was doing a quest which require to enter a specific area, so i reached it clicked on the entrance and i had an entirely black screen intstead of the loading screen.
Then i spawned into the dungeon and i couldn’t use my skill anymore.
I tried every thing, plugin/out controller etc, switching keyboard language, nothing could help.
Potion is working, and we can press the button for the mount but that didnt call it.
Favorite part about this is the bug only occurs while I’m hardwired into my PC. If I switch to Wifi it doesn’t happen but I get really bad lag and rubber banding. So it still unplayable either way.
I have this happen as well. It usually happens when I Alt + Tab. or when I use the Windows key, or when I use the “Snip & Sketch” tool in Windows
Perfect, thank you, I used the map to teleport back to town and turned in a quest. It fixed it instantly. Thank god I don’t have to restart this dungeon lol.
This bug just cost me a few lives and my highest sigil near the end, so no completion for my time wasted. So stupid.
how is this not fixed yet?? probably one of the most gamebreaking bugs currently. do something blizzard
Same thing. EXTREMELY frustrating.
I crafted a tier 15 sigil, used it, cleared the dungeon, and was just about to fight the dungeon boss, when my skills no longer worked. One random mob killed me as I ran around trying to get my skills to fire. Was able to port to town using map, and port back, but skills still wouldn’t work.
When I logged off, I saw that the Battlenet app was updating, so this may be a bug caused by Blizzard pushing updates without notice.
For those of you playing hardcore, I would stop and play softcore. Diablo 4 has WAY too many bugs to risk playing hardcore.
The 2nd biggest factor I’m starting to see from people’s posts is possibly loading screens, so it might be that somehow in conjunction with alt-tabbing that is causing it.
EDIT:
I just noticed this in yesterday’s patch notes, so fingers crossed.
" * Fixed an issue where control inputs would stop responding when a player teleports or zones in and out of a dungeon."
If that worked, i wouldnt be facing this issue. they managed to break something again between now and then. Though its blizzard why do we expect them not to break something when they try to patch bugs out?
I have hit this problem for the first time today. All skills disabled, but if you press it says “that skill requires a different weapon” - cannot execute any skills. Tried reloging, restarting, repair, teleport, engage with NPC’s, created a new character (where the skills do work, but not when you get back to the main). Head in hands now as unsure what to do. The character is unplayable currently 
I’ve now even uninstalled Battlenet / diablo - reinstalled it and its all the same. Unusable skills I cannot action at all.
It cannot therefore be local - there must be something server related to this…
And a further update - 3 characters now locked with this damn skill / action issue.
Now I am running with 2 monitors and alt tabbing out as I do skill progression. It appears to be repeatable if you switch to another monitor then switch back into the game - when the skills lock out.
Going to turn one monitor off - and alt tab in the same monitor to see if it repeats itself…
I’m also seeing the werewolf and werebear glyphs don’t seem to actually add any damage difference and if they do it’s definitely lower then the glyphs claim and it also needs to be addressed.
I have now given up. Single monitor, no alt-tabbing, new character - get to level 20, boom skills locked after a teleport to central location and when you go outside of the town - cannot active the skills. I can talk to NPC’s, I can teleport, i simply cannot fight anything. Apologies to venting in the forum, but this game is unplayable as all time you invest is just lost with no means of re-enabling the char. Any customer service request just comes back to asking me to update this forum. So I before I log off for a month and hope they fix it, the scenarios are
Shame, game looks good but I’ve lost the faith. Hopefully they find the resolution to the issue, will watch the thread, but not going to waste any more time on this game, money down the drain.
– After writing this, couldn’t quite give up… went to the Purveyor of Curiosities. Did a random weapon. Equipped it into main and then replaced it with another. Hey presto the damn skills re-enabled !
AFTER NOTHING WORKED, I NOTICED MY WEAPON WAS GONE. DOUBLE CHECK.
I swear I didn’t delete it/sell it, and I only ever dismantle everything which obviously doesn’t affect equipped gear. On top of this, lately i’ve been exiting safe zones on horseback, but I cant sprint or attack-dismount in the open world until I dismount and remount.
So lame. Perhaps I made a moronic, ironic mistake, but I really have a hard time believing that.
edit. This was a newer Necromancer and I am wondering if i picked up an offhand while I had been using a two hand, equipped it in town (therefore unequipping the 2 hand), and then dismantled the 2 hand. /shrug