1990 Oldsmobile Cutlass Cranking but won't start.

I obtained a '90 Oldsmobile Cutlass Ciera SL from my grandmother for free.

She had not used it in a few months, but before that it was running with random stalls.
Since I've had it, we've had it brought to our mechanic friend who did a once over and changed the brakes, spark plugs, and switched out the catalyctic converter with a straight pipe because it was rusted out, then declared it mechanically sound. After that, it sat in my back yard for about 2 months while I scraped up the funds to get it road legal. Twice a week, i'd start her up to be sure it still starts. Every time, it started fine.

Now, It's road legal. I drove it around for about 30 minutes on a short errand, and after one trip to a store, it refused to start again. It cranked and cranked and cranked and refused to start.
we brought it to a mechanic who ran extensive tests and said that he can't find anything mechanically wrong with it, every test he tried showed good readings, and the computer was giving him no errors at all. He deduced that it probably is the computer. (Not sure why). So we bought a computer for it, the ECM, and he swapped it out - this apparently made the problem even worse and it was not even cranking much. He's given up and now we are at a loss. It's the only vehicle I'm able to get my hands on right now and I need it running!

any help would be appreciated. Thanks

edit - to add, is there any process to replacing the ECM behindthe glove box? I do not see how replacing it with a new one could make the problem worse, did the mechanic possibly do it wrong?