Make and Model - 2001 Nissan Altima Limited Edition GXE

I was driving home and my car felt like it was about to stall. I could barely accelerate. I stopped and turned it off. Took a couple tries but got it started again. Ran fine the rest of the way home. I noticed an odd smell when I got out of the car. Something I never smelled before from my car. It would not start at all after that. It would turn but not fire up. I sprayed starter fluid into the air intake but nothing. Got it towed into a shop. The mechanic said there is spark but absolutely no compression at all.

What could it be?

Edit: Okay, the mechanic called me today. I wanted to clarify what he meant by no compression. About 30 pounds he said I was getting. He told me that because I was able to start it up again that the problem is leaning towards timing. If that is the case, the total labor and cost will equal to about $600. He also said that it might be a head. The heads alone cost $600.

I paid $2500 for it. The car has 150,000 miles on it. What do you guys think?