Oh

Yep, pretty much same thing here. Game froze. I rebooted everything to come back to day 1. Map is there but all building / progress ... gone.

bugs reports sub-forum that way ----->

I disabled steam cloud saves for Valheim. I was playing solo deathless and had managed to get past Moder. Still deathless, I had fully upgraded my padded armor, my frostner, and black iron shield. I was ready for Yag. I log in and immediately get a prompt that "Steam cloud is syncing" (which shouldn't have been the case). I panicked, and closed steam. When I logged back in my character had been replaced by a same named low level red headed character wearing troll hide that I had played with steam cloud enabled months ago.

Steam cloud has ruined my experiences in many games (Grim Dawn, Titan Quest come to mind). I despise the steam cloud.

Not sure if steam cloud was a factor in your problem.

Hopefully someone here on these forums has info that can help you get your stuff back, or at the very least keep it from happening again.