I try to turn my lg50pj350 on and it trys to power up 3 times but nothing happens

The only way to diagnose is electronically by asking an auto electrician to scan it with a pro grade scan tool computer. They can also record what is happening on a drive! It sounds a mess for sure, mechanical and electrical and the computer can diagnose both.

https://www.google.com/search?q=mobile+auto+electrician+near+me

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Is the security light coming on for 3 seconds then going out or does it stay on?

Check to see if you have voltage to the starter if you do try tapping the solenoid while someone cranks it if it starts change out your starter if you have no voltage to your starter start to look for a fusible link that goes to your starter chances are its blown. A fusible link is going to look just like a regular insulated wire but acts as a fuse and blows just like a fuse check the wires to the starter I usually pull a little on the wires to see if the insulation will stretch if it keeps stretching then that's the fusible link that's blown and will need to be changed out. I have seen this on many vehicles where everything works but the starter will not turn over. sometimes the fusible link is located down by the starter and other times its under the hood next to other fusible links you just have to find it let me know how it goes.

Only thing it could have been in the first place is the wire from the switch to the starter is disconnected from the starter. Even a new switch won't fix that. Just get under the car and plug the wire back onto the spade connector.

It may be the starter relay or even the starter. I have had both happen on these cars. a less common problem would be the ignition switch. The starter relay is easy to replace, it is under the hood in the fuse box. Should be listed as starter relay.

if nothing happens when you turn the key the problem is most likely the ignition switch and housing(had mine replaced 3 times already. firstr time at 12,000 miles) what you can try is hit key with rubber mallet while trying to turn but this would only be to get it started so you can drive to a chevy dealer(dont waste your time at any old repair shop for this). Hope this helps.

There is a problem with the theft system. This is the system that pervents the car from starting if the ignition lock cylinder is damaged when someone trys to steal the car.
You can try and do a relearn, it may let the car start after this relearn. The most common cause for this concern is the ignition lock cylinder, it has a computer sensor inside that goes bad, you have to replace the cylinder and program it.
To reprogram/relearn, put on the parking brake(this will keep the headlights from comming on and draining the battery), try to start the car, leave the key in the "on" position. The security light should no be flashing. Wait for tem mins, the light should turn off. At this time, turn the key off for 2 seconds and try to restart. At this point, the car may start, if not let the key stay in the "on" position again for tem mins untill the light turns off. Again turn the key off and try to start the car. It may take 3 times for the system to relearn, if you try 3 times and still no start, you will have to replace the ignition lock cylinder.

Check your car's battery, some time when battery goes bad ( no power ) key will not come out, try giving it a battery jump start.

sounds like the starter, i dont think its siezed the motor in that short distance. try and hit the starter with a hammer or something with someone turning the key at the same time, it should start with acouple trys. if it does than u need a new starter.

if your car is 2003+ try turning the key to run (but not crank) then to acc. 3 times and the engine codes will show up on the odometer, then you can look up what those codes are to help diagnose some of the problems.