I've never installed a chip on a mustang, but i've done my other car.
i'll try and help the best way i can until someone chimes in..i hate messing with ECUs, it makes my heart beat.
To me, it sounds like you didn't put the chip in right.. If it's anything like my old car, and pretty much if its anything like any computer chip, i think the chip should have a tiny little notch on it that needs to be facing the tiny barley noticable indent on the ECU.
Also are the pins on the chip straight?
did you try putting the old chip back in and see if the car starts?
Did you ground yourself before you touched the inside of the ECM?