Ignition trouble?

I have a 2004 Pontiac GrandAm, 4-cylinder automatic, 35000 miles. A couple of months ago, my car wouldn't start & the mechanic replaced the ignition coil, serviced the fuel injection & had the standard tune-up done. The car didn't start again last week. I took it back to the same mechanic & they reset the computers or something because they said something triggered the security system that caused it to not start. (This is whatever security is set in the car standard as I there is no car alarm installed.) They told me if this were to happen again, I would have to replace the ignition. Today, the car had the same problem and did not start. Any idea what the problem may be & how much I should expect to pay? I would like to know if I should trust & go back to this mechanic or find a new one. Thanks.