sounds like the clutch/gearbox has broken and it's welded the input shaft of the box to the end of the crankshaft

happened to me on my sierra and it took several attempts to jump on the gearbox with all the bolts undone before it split from the engine

both were out of the car and on the deck so we had plenty of run up

or it could be something as simple as the clutch has gone bang and one of the springs has snagged on the flywheel

pull the starter out first to see if you can spot any obvious damage