1993 Pontiac Sunbird Timing Belt

Recently while driving this 1993 Pontiac Sunbird. We had been driving on the highway and everything was fine. As we got off on an exit ramp, after letting off the accelerator, the car stalled. After several attempts at restarting the vehicle (unsucessfully). We had it towed home. I tried spraying starter fluid into the air intake to see if it would run, even momentarily, to isolate a fuel feed problem. No luck. I then removed a spark plug and turned the motor over, to find I did get a spark. I then lossened the timing belt cover to see if the belt was in place. It was, but when we turned over the motor the belt did not turn. Would this indicate a "locked up" valve train? Or should I next remove the entire cover to inspect the teeth and or belt on the crankshaft. I hesitate to completely remove the cover and replace the belt. If it is a catastrophic valve failure. It is a lot of work to remove and replace belt, if not a probable fix? Can you advise me please?