I'd start by shifting into neutral - perhaps a space between ranges - to see if the starter will turn the engine over. If not try rotating the engine by hand, perhaps using the front crankshaft bolt with a socket and breaker bar. This will let you know if the engine is seized.
/ Starter motor is knocking loud no engine turn over! Help!
Starter gear may either not be meshing with engine or the solenoid that pops out the shaft is not going out far enough.
If you can turn the engine a little safely and then try, it would rule out engine seized and starter gear not meshing in one go.
Make sure you are in neutral during any of this testing.
/ Starter motor is knocking loud no engine turn over! Help!