2004 silverado cranks but won't start

How long since a tune-up? Check for applicable trouble codes? Make sure battery has a full charge and battery connections all good.
If it cranks good but won't start, have a helper crank it while you check for spark at the spark plugs. If spark everywhere, use a gage and check proper fuel pressure, also check fuel injector pulse. What about your security system?

sounds like head gasket failed can try replacement again but check the head and there machine work was done for crack / damage

Are we talking about the engine does not crank, or does not start ??
If the engine will not crank and the key lock is not working right, it could be the linkage in the steering column or the lock cylinder.

I had the same problem . on my 2002 silverado it turned to be the fuel pump failed because the fuel filter was clogged. I did the work myself could saved myself 219.00 and labor if I replaced a 7.00 $ Filter .

Pump or injectors may be bad .

THERE IS NO WAY THE MOTOR IS GOING BACWARDS AND MAY BE YOU SHOULD ASK YOUR DAD FOR HELP.

I had a similar problem on my older chevy truck and it was the pickup coil which is found inside the distributor. I'm not sure what the 2004 ignition distributor looks like, but if it is the same style as the 1990's, then your problem most likely lies within the pick-up coil and/or ignition module. Ask a parts store if your distributor has those parts in it and if so, I would replace both at same time. Goodluck, Paul

Have you had the vehicle checked for DTC'S - diagnostic trouble codes ?.

they might have not put the proper gap on the plugs..and might have over filled the engine with too much oil..