The 1994 & later US model C-Class have a starter interlock relay!!
On cars without an alarm system the central locking system MUST send signal to this relay before the starter will engage.
If the car has a factory alarm then the signal cones from the alarm ECU.
Next time that this happens TRY locking & unlocking the car several times & then see if the starter will work.
ALSO the central locking system MUST work.
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