Are you talking about the large center chronograph seconds hand, or the small subdial seconds hand?

Many quartz chronographs have a function where the large center chronograph hand stops if left running for a long period of time, sometimes 1 hour, sometimes 12, etc. This is to preserve battery life, and they aren’t meant to be left running continuously.

Should be covered by Warranty, most likely they will give you a new watch Vs wasting time trying to fix a $50 dollar ETA quartz movement.