Wont start but it turns over dieg.ok on 2002 avalanche

You need to have the factory security system scanned, it is most likely locking you out due to a fault.

your ECU is playing up, probably thinking that it is stolen (security light) needs to visit a dealers computer, ring your dealer he may be able to reset for you

have you checked the fuse ?

probaly a broken rear/lower planetary set or ring gear is broke

Check fuel pump relay under hood and fuse.If they are good it is going to be the fuel pump.

I would have the computer scanned for trouble codes next.

Those are known for having the fuel pump go out. Take the gas cap off, open the little steel door with a screwdriver and have someone turn the key forward, not cranking it over, just turn it until all the dash lights come on and see if you can hear the fuel pump kick on for about 2 seconds. You definitely need 2 people to do this. If you can hear the fuel pump then its probally not getting spark.

Do a bench test on the starter. Remove starter, hook up a ground cable to starter, and positive to two prongs on starter and it should spin. You can do this in vechile too but not suggested due to risk of injury. If it doesn't spin, probally a solenoid or the starter it self. Unlikely but possible.

i have the same problem my avalanche wont crank, and the fuel gage dont go up, when the fuel gage works the truck starts but its been two days and it wont crank plz help...

alternator with a built in voltage regulator. can short out a kill the battery, even just over night.
pull the alternator and have it bench tested at autozone or kragens.