Starting the engine sometimes works sometimes doesn't?

[David Symons]

For no logical reason, hot or cold, the engine does'nt fire up! I then end up exiting the vehicle; locking and unlocking; which sometimes works and sometimes does'nt.

[Magnet]

Thanks, could you please complete your profile, so we know what vehicle we are dealing with, and who,  and where you are. 
Many thanks.