Not playing with Steam cloud, in the /Irongate/Valheim/ folder, there are two subfolders, /characters/ and /worlds/. Usually there are .old files in them in case the currently used character/world get corrupted. I don't know if you can access these files or if you already tried it, but in case you didn't, just remove this ".old" suffix from their name.

Next time, save those files on a regular basis in another folder.