The most likely cause is an internal electrical fault, not the batteries. Since the flash is either completely dead or fully normal, likely suspects are:

• poor battery/contact continuity inside the flash
• a faulty internal connection
• failing internal components such as swollen capacitors

If the unit was very warm when it shut down, thermal protection is also possible; some flashes temporarily disable themselves when overheated and work again after cooling. That’s more likely if you were firing rapidly at high power.

What to check:

• inspect and clean the battery terminals/contacts
• make sure the batteries fit tightly and the door closes firmly
• if you’re experienced, check continuity with a multimeter

Be careful: flash capacitors can hold dangerous voltage even when off. If you’re not comfortable opening it, the safest option is professional repair or replacement. Given one report of swollen capacitors, an internal component failure is a strong possibility.