The files for your hotbar has most likely been corrupted.
Head into the Steam folder in your file explorer, then locate the tower unite game folder, there’s a specific folder you need to access called “ClientPlayerData2”
From the steam folder the path to the specified folder should look like this:
~\Steam\steamapps\common\Tower Unite\Tower\ClientPlayerData2
Delete every single .dat file within said folder and then the issue should be fixed when you relaunch the game.
If steam tells you certain files are unsynchronized tell it to upload the files from your local computer.