Not Starting Chevy Blazer, S1,1998, 109k miles, V6

Last year my car would not start, 5 months ago. I called AAA, they came to give me a jump and it would not budge. Had the car towed to the mechanic, lights were working, radio, just no turning over.

When it got to the mechanic, he cranked it to get it moved from outside to the diagnostic table and it started fine. He could not get it to not start again. He ran diagnostic, all was working fine. No problems. After having it for a week, he changed all the plugs, old wires and gave it a tune up. He thinks it was the alarm system but the deal is I never used the alarm when I got the car, the push botton gadget never worked and I simply took the battery out and tossed it.

I would hate to continue putting money into this truck w out knowing what is really wrong w it. IT did not start again today, and I'dhave to have it towed again. Lights, music and everything inside the car is on. Just would not click over.

Could it be the alternator, switch or something else?
I had the themostat and 4WD switch replaced last year, don't know if that is correlated.