maybe it needs lowercase letters
Bentley is absolutely right... check the fault code provided conveniently for you in the Processor Status window of RSLogix 500 and that will give you some hints.
When you go online with RSLogix 500 and the "Run/Prog" indicator reads "FAULT", you can click on that indicator to get a drop-down menu which includes an option to "go to error". That's the quickest way to have the software direct you right to the error code that has caused the controller to enter fault mode.