I have a 1995 C280. My car did the same thing. Started fine when cold, but wouldn't usually start when warm. Always take a couple turns of the ignition before it finally started. Is your engine still turning when you do this?
Then your problem is the Overvolt Protection Relay. I am 95% sure that will cure your problem. This is a VERY VERY common problem. The new ones have been upgraded to fix this problem. You can get them right here on partsshop or fastlane. Good luck and be sure to let us know how it goes!

__________________
2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)