I've regularly seen this error occur when the Fanuc control has been turned off whilst waiting for RS232 input (or in the process of RS232 progam download).
I quickly learned you should NEVER turn the Fanuc control off whilst you have LSK or INPUT flashing in the bottom of the screen. Always press the RESET key to quit RS232 mode before turning off the control.
I don't know if this is what has happened in your case but it is always a good 'rule of thumb' to follow anyway.
Saying that, I also remember a number of occasions where I have LSK flashing and the control has 'locked up' so I have no choice but to kill the power and hope I don't get the dreaded PS101 alarm
As Steve says, you've no choice but to do as Martin suggests but you'll loose and CNC programs you had in the Fanucs memory
