Pool heater not starting

The old Hayward H-series kicked on just fine yesterday when I started it for the first time in about 6-7 months, and it ran for 8 hours or so. Today, all I am getting is the subtle “errrr” sound coming from it, and after a few minutes it turns off. I already turned off the master power supply and turned back on with no effect.

Any idiot where to start? I found the instruction/installation manual online, so I can start troubleshooting, but hoping someone knows the issue so this can go quicker.

Thank you in advance!