Have you recently installed a radio? I had one "professionally installed" and found out after two batteries they wired it wrong. The amp was constantly drawing power off the battery because of the wiring of the new radio. Also, if you have an automatic car starter (remote start), make sure the starting switch path is true, not open.
Is it charging, if not have you checked wiring to alternator.
Check the battery cables or starter.
Voltage Regulator?
Is the battery/alt warning light on? if they didn't disconnect the battery while the starter was being replaced and they accidently shorted the battery cable out it could have damaged the regulator of the alternator some of the Delphi regulators are very sensitive to that, but if it tested normal at the parts place, does it run down while driving or while sitting?
You need to replaced the starter or the alternator.
If the engine doesn't turn over at all there are a few things that are possible. Start with the battery ground cable and make sure it is making a good ground and is not broken. Also check the positive cable from the battery to make sure you have a good connection and no breaks in the wire. If these are both good you may need to replace the starter motor. There are brushes inside the starter (yes they can be replaced if you know how to take the starter apart) that will grind down to the point of not making enough contact to start your car. Since you said you had the alternator replaced I'm thinking it probably is the starter because any competent mechanic would have checked the battery cables before replacing the alternator.
check your alternator or make sure the starter is on correctly
Starter Ignition Neutral switch
battery need to be at lease 14 volts to be able to start
how will I know my Voltage regulator is bad. I have replaced the starter and had the battery and alternator checked they are good. My car will start when it is colder outside but as the day gets hotter it will not start.. Can you help
could be the alternator or starter sounds like the alternator if the battery is gone flat get the alternator tested at auto parts or garage jump charging the battery might get you out of trouble for a bit
Check the alternator if that don't do it change the battery