95 Lexus sc400
 

My ECM went out, I sent it off, had it rebuilt, tested, and sent back. I plugged it back in and my car doesn't recognize the computer. Such as I turned the ignition switch to on and the check engine light does not light up. The light does work, but not reading the computer. Any ideas on a problem or solution?

Sounds like whoever rebuilt your ECU *****it up. Tanin electronics who is a vendor here on this site is reputable and does good work. I would sent it to them for an inspection/test.
Also, check for bent pins on the ECU, this will cause issues too.