The machine builder will have a tool number to tool pot table in the diagnostics. Without knowing the diagnostic numbers where they're stored it could be difficult to determine which tool is currently assigned to the broken pot. In that case do this.
MDI T0M6
This will call tool zero to the spindle.
If this brings a tool to the spindle, remove it.
Now manually rotate the carousel until the broken pot is in the tool change or active position.
Now MDI M6
This will now install T0 into the broken tool pot.
Now never program or use T0 in your programs or daily routines. It's likely that you don't anyway.
If you know which diagnostic parameters control the tool pot assignments, you could change the broken pot to T Zero and be done with it that way.
You will not be able to set the machine up to create an alarm condition when the broken pot is called, as this would have to be designed in the ladder, so this is about the best you can hope for.