1. Add -console to the Valheim launch properties.
2. In the game, press F5 and type: devcommands
3. You can use: spawn [itemname] [amount] to get back what you lost.

Note that this only works for the person hosting the game and not the one joining you in non-dedicated server co-op.

Full list here: https://valheim.fandom.com/wiki/Item_IDs