Maybe the wire has come off the starter terminal again. You should be able to see it. It could be simply that the little brass connector on the end of the wire is damaged or loose. If you turn the lights on, are they bright? If they are not, then I'd say you have a dead battery again. If the lights are bright, then the starter is simply not getting power from the ignition switch.
When the mechanic replaced the starter, the wire terminals may have been damaged just by taking them off and putting them back on. When you were driving, one wire may have come loose and so the car stayed running, but now won't start.
I hope you find an easy solution to your problem!
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Rob Bareiss, New London CT ::: 86 244DL, 87 244DL, 88 744GLE, 91 244: 808K total