If it's capable of ticking over correct at some point then I wouldn't think it's anything mechanical like piston, valve etc etc.
It will take a bit of digging to get to the bottom of the problem and if your not very handy at cars you will just have to look at getting someone else to do it for you.
Did this just all happen 1 day all of a sudden?
Had you done anything on the car prior to it ? Anything? Serviced it change plugs etc etc