2000 Mercury Cougar Won't Start

I tried a post before but no one answered it. Last week before X-mas my car would not start. It would crank (ie turn the key hear the ra-ra-ra-ra-ra-ra-ra sound but not turn over and start. I an NOT a car guy so I hope my terms are right) I had it towed to a dealer in the morning a day later since it would still not start. It was in a garage while it was cold outside I have not have any problems in the past. The dealer called about 6 hours later saying the car started just fine and their battery check was normal and the alternator and starter was fine. It was a sunny day and about 40 degrees. I picked it up, ran fine. I even filled her up with some new gas and things were fine. Four days later I go to start the car in my garage and nothing. Again it will crank but not start. My buddies moved it outside the garage and moved it around a bit to maybe simulate the motion of a tow truck but again it would not start. I want to know what to tell the dealer to look for when I bring it back in on Friday. Any help would be great.