READ HERE : World gone / lost / reset after 0.209.X patch
We are very much aware of the patch we released when we overhauled steam cloud sync and some other saving cause issues for some players.
For now, if you have a low windows cache memory or low RAM (4-8Gb) and you have multiple things using your memory in your PC while also playing Valheim. You might run into issues with memory being filled and the saves taking so long time, that windows things Valheim isn't responding anymore. You can usually click "Wait for valheim to respond" and it will eventually stop being stuck.
But for those of you who already lost a world. Our nice friend MayaOmkara over at Reddit have posted a guide that is very indepth on how to make your world come back again :)
(And yes, we do have an awesome community that works together with us to help alot of you players out there with issues and problems. We couldn't be able to help everyone without you all)