What is wrong with my car?

My 1995 Nissan Sentra will not start. The engine cranks/grinds like it is trying to start but will not turn over. I have a brand new battery, new distributor, new spark plugs and all new belts. Just got an oil change. Everything that I think could be causing the problem has been looked into; it has done this before and at first we thought it was flooding but when it continued and when it would not start even after leaving it alone, we have taken it in and had all the above parts replaced. What else could be going on and what can I do? Thank you to anyone who can help!