1993 Cadillac Deville Timing Belt

93 Cadillac Deville 115000. While speeding up on interstate this morning, I heard something close to a lose pebble bouncing underneath the carriage. Suddenly the Charge light comes on in the message center. I put it in neutral and turn the ignition, but realize that the engine is still running. I go for about 4 more miles (with no power steering at this point) into my parking lot (last 1/2 mile the Eng Temp warning comes on). Shut off engine and pop the hood. I see that the timing belt has become dislodged. I look at the belt and all seems in good condition. What could have caused the belt to come loose? Should I re-align the belt and hope for the best or has one of the "?Engine wheels?" Lost it's physical setting to let the belt slide off of track?