Why Grand Prix WON'T start, NOBODY knows why?

I blew a head gasket a while back,
had a trusted mechanic friend
replace it. It started for a second,
then shut off. The security light kept
flashing and simply WOULD NOT
start! He replaced some sensors,
checked over everything 10x over,
ran diagnostics, checked fuses- no
luck! We towed it out to a group of
mechanics who had it for about a
year- working in it when they could..
Disassembled and reassembled all
sorts of stuff and FINALLY found a
pintched ground wire. They DID get it
running, although the security light
remained on, and at this point, it had
sat for so long- who knows what
went bad! They said to drive it and
"clean it out." We drove it a little for a
couple weeks, things were a little
rusty- but working! (Still with security
light) One day, my husband parked it
for about 20min. When he went back
to start it- it wouldn't! We had it
towed, put fuel injector cleaner in the
gas tank: nothing! It sounds like it
wants to start but Won't turn over.
Any suggestions?Also- before the
original problem: I
was experiencing issues with ALL of
the INSIDE lights randomly shutting
off on me! All the outside lights were
still working. Had fuses checked, but
everything seemed fine. The lights
NEVER went out when anyone was
looking at it- mysterious :) didn't
know if it could be related