I’ve seen this happen a few times. I join a group through the Dungeon Finder. Eventually I look back at my screen, and I’m no longer in the group anymore.
There’s nothing in chat about:
Just suddenly I’m not in a party anymore.
What causes this?
Have you ever seen a message saying you were removed from the group? If not, there may not be a vote kicked message I can’t remember.
Are you (a) still in the instance? Or do you suddenly find you’re (b) back where you were before you got teleported to the dungeon?
If (a), everyone else in your group dropped group and you are alone. Of course, in this case, you’d be party leader now, and if you used the group finder, it will re-queue you to get four other new party members. You have to wait a bit, but you can continue.
If (b), you were removed from the group for some reason. You can re-queue, but if you didn’t down at least one boss or wipe at least once, you will probably get hit with the “dungeon deserter” debuff and you’ll have to wait a half an hour.
Either way, your chat log (lower left corner) should be giving you information on what’s going on in any of these circumstances. Make sure the “General” tab at the top is active, not the combat log.