Ford Repair: Car Trouble, car garages, ford taurus

Question
I have a 2001 Ford Taurus that has been giving me problems on and off for several years now.  Sometimes it starts right up and gives me no problems for months, and other times (like right now) it won't start at all.  I've taken it into 3 different car garages and no one can figure it out.  When they hook it up to diagnostics no code shows.  It's not a dependable car at all and I'm wondering if you know what it could possibly be? It doesn't matter what season it is, it will not start in any of the months, cold or hot weather.  It seems to not start more often when I shut it off for a short amount of time like when I get gas or run into the store.  Any help/suggestions would be greatly appreciated because I don't know what else to do!

Answer
Aubrea, there are many possibilities of why this happens.  Please let me know what do you mean by "Does not Start", engine turn over, but will not start....  or no response, or some noises.... I need more detail of the problem to able to help.  Thanks!