Won't Start After Being Driven

I Have A Galant 2.4 L 4cyl With 107000 Miles - (Engine Hot). What Could This Be? After I drive the car, park it and turn it off-it won't start back up! It just keeps tryin (engine turning and turning and turning) but won't start. It has been doing this for a couple months. Back then (a couple months ago) it would start up again if u just gave it a couple hours, BUT now, it takes it 4 or 5 hours. And sometimes longer, before it will start again.

I should have added more info in my original question, sorry about that. When the problem first started happening a couple months ago, there was also a strong oil scent inside the car. The valve cover gasket was bad-allowing a tiny bit of oil to seep out around the valve cover AND 2 of my 4 spark plugs were completely fouled with oil. I replaced the valve cover gasket (it no longer leaks) and cleaned the plugs with brake cleaner since they were only a month old. Is this okay? Should I have replaced the plugs anyway?