C230 2005 will not start

If it fails to turn over then check on battery condition and on battery connections (do full beam headlights stay consistently bright for ten minutes). If Ok then check on wiring to and from starter motor and on condition of starter motor brushes. If turning over can you hear the fuel pump running; whirring sound when key at position prior to 'start'. If no fuel pump check on fuse for pump relay, check on relay, check on voltage at pump. If pump supplied with voltage but not running give it knock to see if you can get some kind of response from it. If completely dead replace pump and filter assembly.