Nah, that's by design, not a malfunction. The UV sterilizer has an automatic safety cycle. After every 4 hours of continuous operation, the UV light turns itself off.
You can turn it back on, but only via the remote control. There's no button for it on the main control panel. Just hit the UV button on the remote.
If it's not coming on at all, first make sure the purifier itself is on. The UV light starts working after the main appliance is turned on. Also, check that you're pointing the remote directly at the unit's control panel.
If that didn't work, and the main unit is functioning fine, the UV component might have an issue. Since it's an internal part you shouldn't refit or disassemble, your next step is to contact Mooka customer service for advice.