You are sure you loaded the correct world in menu?
=>If yes try to relog/start the game sometimes it is just not loading proberly.
=>If this didn't help you can check the world save directory if your world file even exists any longer. something like */user/AppData/LocalLow/IronGate/Valheim/world

If the file exists you can try this but never delete a file just rename them! Otherwise when something goes wrong your save could be deleted.
disable steam cloud
open AppData/LocalLow/IronGate/Valheim/world
rename "yourworld".fwl.new -> "yourworld".fwl ..... simply just remove .new on the end. you can keep there these .old files
launch game and start your world on world list
exit game.
turn on steam cloud
start game

If there is no file at all your world and progress are lost.
Errors like this happen a lot sadly there is no fix from devs yet.
Should you not play on your own pc you always need to save the file manually after your playsession to save your world progress to a stick!