Copy out saves folder and name it as backup date it.
The 2 fodlers most important is Charactors and Worlds. If a problem occurs copy them back and replace others.
C:\Users\YOURNAME\AppData\LocalLow\IronGate\Valheim
Yu should be keeping all your gear when going from MP to single player, Log Off to create a save point.
If you just quit it wont create a save point very likely is your problem. So always log off