If electrical (you turn the key and there's no response at all) -
I would check the connection. Make sure it's hooked up right. If so, use a test light to check the circuit while someone else turns the key (Park or neutral for 5speed with ebrake on [safety] or on truck jackstands on level ground [more stable]) - maybe a short or bad relay or fuse. Use test light along circuit until the light goes on, then look at what was before that spot. There it is.
If mechanical-
If you hear the starter motor trying to work- either battery is bad, or there's something causing mechanical resistance inside the motor (see if a 1/2" ratchet will turn the motor- if you can turn it by hand, then it's the battery)
[guess work]
BAttery?, starter? ignition switch?
check the solenoid, and the starter.
There are several reasons why a 1999 Blazer S10 may not turn over. Some reasons could be bad spark plugs, dead battery, or even a bad starter. A mechanic will be able to determine the cause.
Corroded battery cables. Other than that you probably need to remove the starter and have it tested.
Engine seized? Bad starter? Bad starter solenoid? Dirty or loose battery cables? Bad neutral switch?
If a 94 Thunderbird starter will not turn over, the battery may be dead. The battery should be charged with jumper cables.
if the battery and battery conections are good, then you need to replace the starter motor
Start with having the battery tested then be sure the battery connections are clean and tight. Check the starter relay. Try moving the shift selector from park to neutral then back to park. If the starter still won't engage you may have a faulty starter.
Dead battery, corroded or loose battery or starter cables, defective starter or starter solenoid, loose ground connection.
- dead battery - starter motor defective - low battery - ignition key problems starter, ignition problems
I means you have a battery problem or a starter problem or dirty/loose battery cable connections. If the battery is operating the starter and the starter is turning the engine over but the engine won't start and run, you may have a spark problem or a fuel problem.
The starter may not have been properly installed, or it may be as simply as one of the power cables to the starter having come loose, and preventing a circuit from being made.