I can't turn the ignition to start my 1999 300M what do I do?

Sounds to me like you have a broken linkage from your ignition tumbler to your ignition switch. It is a plastic "arm" that pushs.pulls a metal tab on your ignition switch.

The chatter is due to low voltage. Check all the connections. The battery may have corrosion that lowers the voltage.

see the diagrams and troubleshooting flow chart: God bless you

You don't assume anything

You recall how old things are & then test
the easiest & most likely items

Battery
Cables
Starter Solenoid

Did you ever figured it out on the GS400?

possible ignition module or fuel pump both have problem when they get hot or warm. try to smack the fuel tank with a hammer or equivalant. when it wont start if it runs then replace the fuel pump & fuel filter & fuel relay!

i know industry jargon can be confusing, but lets try this. if it turns over but won't start, it could be your ''ignition module'' that sits on top of the vave cover on the motor or it might be the fuel pump or filter. make sure when you crank the engine that the key is all the way turned or it may not always make a proper contact to activate the ignition module. iv'e seen this before & had to change the ignition switch & all worked ok after that. i would need to know engine lt size to give you more details on the matter. please get back to me.

Your Sonata is equipped with a "Keyless Entry & Burglar Alarm" system. When the Burglar Alarm is activated, it KILLS the starting system through the used of a relay called the "Burglar Alarm Relay". This mode prevents engine starting with the key.
Solution #1
Disarm the system with the Keyless Remote (key fob) If you don't have this remote (or its battery is dead) replace the battery & try disarming again.
Solution #2
Enter the vehicle & insert the key in the ignition and turn the ignition switch to 'RUN'. Wait 3 to 5 minutes until the alarm stops. You should then be able to start the engine.

Have a qualified/certified Hyundai Service Technician properly diagnose this issue.

any chance it was the timing belt that let go or crank sensor?

maybe the ignition wasnt your problem. http://vatspasslockpasskeysecurityhelp.yolasite.com/