Give the forum more info. What car? Year?
What did you tap into to get power to the amp? That's the likely cause. Check all power/ground connections you made. You may need to retrace your steps and undo any changes you've made (i.e. disconnect the amp and wiring to see if car starts)

If it cranks but is not starting, then the starter and battery are o.k. I think you have something wrong in the ignition system wiring.