Characters still there: no stash, no adventure mode unlocked

Hi,
Trying to come back after a long time away. I still have all of my characters (and achievements), but no one has anything in their stashes. (No messages saying I have seasonal stash items either, but one of my characters hasn’t been a seasonal character just about ever, I think.)
Additionally, I’m locked to doing the campaign from the beginning on all characters and am unable to go into adventure mode. Please advise!
Thanks in advance!

It sounds like your Heroes were created and played as Seasonal.
When the Seasons ended, they reverted to Non Season and you had 30 days to claim the mail that contained the stash items.
If you missed the deadline, the items are gone.
Adventure Mode is unlocked during Season but you must complete the Campaign in Non Season to unlock it there.
If you complete the Campaign in Season, the unlock will carry over at the End.