All EX-switches are "virtual-chassis" aware per default so even that you haven't used virtual-chassis it's running in the background.
You could try to regain master role via 'request virtual-chassis reactivate'.
I can see that the switch is running 15.1X53-D51 which is known to have multiple bugs. I would recommend you to plan an upgrade to 15.1X53-D58 as soon as possible.... when you have regained proper operation of the switch.