2000 4.6l on 2000 Ford Mustang

my car backfired last night when i attemped to start and now it will not sty running. and pops even few attempts. What is causing this?

Asked by for the 2000 Ford Mustang
do you have any warning lights illuminated? (is the check engine light on?)
do you know what the trouble code is? I will try to help if you have the specific code
Code P0442 is "EVAP Control System Small Leak Detected"

This can be a difficult code to chase down on your own. Most smog repair shops have a "small leak detector" or what many techs call a "smoke tester". Smoke is injected into the EVAP system and the shop uses a very bright light to watch for the smoke.

The description for this trouble code says a loose gas cap can cause this code to be set but I kinda doubt it would also cause the backfiring/stumbling problem. Make sure your gas cap is all the way tight and not accidentally cross threaded. If the cap is tweaked slightly it won't seal completely and lead to this 'small leak'. Smog shops here in CA have a machine that will test gas caps for sealing ability if you're not sure about your cap's condition.

Simple visual checks you can do in the engine compartment include making sure your air intake tube doesn't have any cracks or holes, no dangling or melted vacuum lines. If you're comfortable doing this, you can spray brake clean around the intake gaskets and hose fittings and see if the idle changes. Many techs use this to chase down leaks quickly.

There is a service bulletin from Ford that describes the diagnosis procedures for this DTC (diagnostic trouble code) but the first step is to connect the smoke machine and scan tool.

hope this helps, I wish I had an easier fix for you.
I should mention that you should be careful about what you spray the brake clean on. Try to not hit wires or connectors and avoid soaking sensors on the throttle body
yes, auto zone said there was a vacuum leak and we could not find one any where-

Code 0442 emission