My ford mustang won't start. It makes a clicking sound and won't turn over.?

You have a dead battery. Get a jump from a neighbor or AAA.

if your battery is good then your starter solenoid is bad depending on the year it is either on the firewall next to the battery follow the positive cable off the battery it will go to the solenoid or it is on your starter 18 dollar part

Jump start it. It'll take about 10-15 mins for it to get a good enough charge and if you keep trying to start it before that it'll just run it down more.

Sounds like a dead battery to me. Was it ok before it sat empty?? Maybe your fuel pump went. Talk to a mechanic or check the battery. If it's not that try a new fuel pump (not too expensive). Good luck

Try to jump start it sounds like you might have a bad solinoid or starter.

Try checking you positive and negative battery cables. If they are not making a proper connection you won't be able to get the amount of voltage needed to start your engine. I had this problem once when I was in high school and I drove my truck to school just fine but when I went out to my truck after school let out it wouldnt turn over. It just clicked repeatedly. So I got in and wiggled the battery cables and it fired right up. Then went home and tightened them. Might not be the case in your situation but that's what the problem was in mine.

Dead battery is where I'd start. If that doesn't fix it, try the starter.

jump it, if that dont help, its either a bad selinoid or voltage regulator/connections

Your battery is low on voltage. You shouldn't have added the gas. The car was getting enough as it was. See if you can get it jumped and then take it to a auto parts store and have them check out the battery. They can do a load test on it. good luck.