Does it have spark, will it start if you spray carb cleaner into the inake,
basic checks here.

As one of the posts to your message mentions, checking the basics is
the place to start.  If, however, you're beyond this point, I may have
something to offer.  This is a full-size car with a traditional GM 60s,
70s, 80s V8 like, for instance, an Olds 307?  Among many possibilities,
the timing chain may have slipped.  I'm personally aware of several
experiences like yours.  Though I'm not saying it's the only thing that
can cause something like you describe, each instance I've encountered
resulted from a slipped timing chain.  High mileage is usually
involved.  My 1981 Pontiac slipped around 210,000.  My Dad's 1981
Pontiac slipped at 137,000.  Something to keep in mind as you rule out
more likely causes.