Hi asdf,
My apologies for the delay in responding to your post. I’m sorry to hear this issue has affected your save.
In case you have not seen it, I have a post on this issue here: [Known Issue][PS4/XB1] Inventory and Tool Belt Reset (Multiplayer). While there are no workarounds for this issue at the moment, if any are found that post will be updated.
When your friend left the game, did they choose to leave or did they get disconnected and not return to the save?
As you are unable to craft another toolbelt, does this mean you were at level 4? Players report being able to craft the next level up but cannot retrieve the slots lost.
When you saved the game and exited to the main menu, do you recall if your axe or any other items were in your tool belt at the time?
Unfortunately unless you had a backup of the save file it cannot be reverted. However the team are still investigating this issue and it is unknown if an update could restore the toolbelt at a later stage.
Thank you for also mentioning the crates, while it may not be related, I will make a note of it.