1999 Pontiac Grand Prix car wont turn over

Are you saying the vehicle cranks good but won't start, or, it won't even crank? If it won't even crank, I'd check the starter circuit. If it cranks good but won't start, have a helper crank it while you visually check for spark at the plugs. If you have spark everywhere, use a gage and check fuel pressure. Some of the national brand autostores used to loan out a fuel pressure gage, if you don't have one.

To check for spark at the plugs, pull a spark plug wire, stick a known good plug or spark tester in the boot. Ground metal part of plug/tester to metal part of engine, watch for spark, engine cranking.

Check for any applicable trouble codes, at the least, to check for codes, you need an obd2 code reader.

Is the theft/security lite on the dash, on or flashing when it stalls. The security system may be disabling the fuel injectors. Have the trouble codes checked, see if anything shows up.

The possibilities as far as no code diagnostics, the list is as long as your arm.