I have a 92 Lexus LS400 it won't start. It was running when I parked it, but its been sitting for about a year and now it won't start. I put new gas in it but i didnt drain the old gas and i replaced the spark plugs and it still won't start.

check your charging system if the alternator is not putting out enough voltage your warning lights will all stay on when running, they are designed to go out over 13 volts

Check and see do lights come on when not running. Most likely a dead battery, but jump it and it goes. Bad alternator not charging battery. No power, no go.

There is no advantage in mixing fuels If the car is tuned to run on regular leave it at that as the computer has been set for that fuel. Check with a dealer and find the recommended fuel for your vehicle . By going to a speed shop that has the ability to reset you cpu to a higher/lower fuel rating you can retune the motor. But if you are running on the recommended fuel leave it alone. It will be cheaper in the long run

Sounds like a major fuse blown.
Any wiring disconnected or stripped near the pump ?
Have you checked the major fuses around the battery and underhood fuse box ?

batteries will show 12 v and still not have enough amps to start the vehicle, clean all the connection points on the battery , and charge it fully, start the vehicle and check again for voltage at the battery posts, you should have a reading of 13.5 to 14.5 volts, if not, yes you have a alternator malfunction,

check power steering levels i have the same year Gen I-III have a power steering defect PS fluid pumps into system then out tail pipe, white smoke.Lexus fixed this in later models.. check lexls.com the solution is there.

gas treatment.

Check your alternator

check the brakes for sticking, will it go in reverse?

my 92 lexus turns over but will not crank, is it the timing or starter