I was playing with my sorc today and my computer crashed and restarted. I logged back in to play some more right after the restart to find that character is just gone. Customer support is useless for this issue.
Try logging out of your battle account and restart your computer. Then log back into your battle account and start d2r again and see if your character comes back. Just give it a shot. It might not work though.
Any time your computer does a hard shutdown (crash, power outage, etc.) while D2R is open will cause the memory to corrupt, this includes your current character and shared stash files.
You’ll need to navigate to C:\Users\username\Saved Games\Diablo II Resurrected and either replace charname.d2s and SharedStashSoftCoreV2.d2i (or SharedStashHardCoreV2.d2i if you were playing a hardcore character) with backups if you have them or delete them because they’re corrupted.
Unfortunately it’ll probably never change because that’s just the way D2R is designed (keeps stuff loaded in memory to improve load times). All you can do is backup your files often and use those in times of crashes.