Engine Turns but will not starts

Hi my name is Serey and I am looking for some trouble shooting help.

I own a 2000 S500 with 139,000 mile plus. Recently, I took my family on vacation and drove from Mass to Florida and back.

Here is the problem that I'm trying to diagnose.

While I was in Florida this problem occured. I drove the car and stopped to have lunch and the engine would not start. It would just turn and turn and turn. My thought was that there might be an ignition system issue since the engine truns but would not fire-up. So I disconnected the negative battery terminal and I pulled the fuse out one by one to see if I have any blown fuses and after about 10 minutes or so it starts just fine.

It started just fine after that until I got back home and the same problem reoccurred 5 or 6 days later but after 6 or 7 times turning the key it started.

Today (4 days later) I drove my wife and children to visit my parents and now it will not start at all. The engine would just turn and turn and turn but will NOT fire-up.

Any ideas as to what could be the source of this ignition problem?

Any help in determining the source of this problem is greatly appreciated.

Thank you!

Best regards,
Serey V. Oum