The following list contains parts I've had to replace at one time or another on cars that wouldn't start, or ran really raggedly. I'm not sure your particular model will include all of them. (I've been driving for 30 years and witnessed many changes in how cars are put together.)
battery, alternator/solenoid, starter, ignition coil or switch, spark plugs, spark plug wires (that were loose), fuel injectors, distributor cap
I even had one friend that had a car with some kind of special "kill switch." He opened his locked door by reaching through an open window and unlocked from the inside. The car started, but about 10 miles or so down the highway, it just slowed and died. It wouldn't start again until the switch was reset.
Might be a bad heater control
water in distributor cap
Some problems which could prevent your car from starting is, maybe you didn't put your keys in all the way. Also, you might need an oil change, or some more gas in your tank.
Your 2000 GMC Sierra Vortec might be hard to start because the battery or the starter are worn or dead. The GMC Sierra might also be hard to start because of a clogged fuel filter.
the security chip in the key might be bad or missing
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Your 1996 GMC Sierra truck might not go in reverse because the transmission fluid is empty. You might also have damaged the transmission or the transmission linkage could be broken.
your switch that controls the compresser might be bad.
Your 2000 4.8 liter Sierra might run rough at an idle because of clogged fuel injectors or a clogged fuel filter. You might also have gotten water in the gasoline.
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It's the Sierra Nevada which roughly translates to Snow Range.
There is some debate as to what problems one might experience with a 2002 GMC Sierra. One of the most noted issues, however, was that with the starter.