C200 Merc 2009 model, engine turns but car wont start, why

Same for all engines. (no engine stated, so no exact answers)
is your gas or diesel (ill guess gas)?
gasoline and not LPG or other fuels. real gas ,nor flex fuels on E100
cranks but no starts.
blown fuses, check fuses first, ( always #1 with EFI)
bad gas.,
the first check is spark , gas dont burn lacking SPark (mr,otto cycle)
watch funny guy SCOTTY.
https://www.youtube.com/watch?v=064Ilsz8Fzg&NR=1&feature=fvwp

any engine can run on test fuel, if it has spark (Otto)
some engines will never start

1. lost compreesion 1 or more cylinders
2. melted CAT converter, blocking exhaust
3. EgR stuck open, wide, if your unstated engine has one.
4. fuel pressure zero or 10psi. dead. must be at spec.