What happens if the pantograph isn't lowered when the loco enters the dead zone?

Usually there is no problem, if the master circuit breaker of the loco has been switched off. In most cases of neutral sections, therefore, the driver does not have to lower the pantograph. If a live loco enters this section without its master circuit breaker turned off, then there is a possibility of sparking or transient disturbances, which can trip protective circuits in the loco and bring the train to a halt. (Rarely, it may trip breakers for the OHE and bring all the traffic to a halt.) Regardless of this, and whether or not the pantograph is lowered, once the loco enters the dead zone it loses power and will grind to a halt once it loses its momentum, if it cannot coast all the way to the next live section.

Source - IFRCA.org