How can I replace my Engine Computer or PCM in my 2003 Mazda Protege5?

What is your question?

Yesterday I had a stranger helping me put in a universal heated O2 sensor, and it was giving us trouble since the wires from Mazda didn't match the universal wire colors. He got out a meter and was testing the wires, but accidentally had it on opposite ends (positive on the negative, negative on the positive) and suddenly after that the car wouldn't start. You could tell the car wanted to start, but no matter how many times I cranked it, it wouldn't fully turn over. Do you think the ECM is fried, or could it just be a blown fuse? And if it is the ECM, how could I get it repaired/replaced?

What makes this problem better or worse?

Nothing I do makes the car run.

How long has your 2003 Mazda Protege5 had this problem?

Two days.