Something is upsetting the DC conditions for the CRT, when the fault occurred and the screen brightened up, vertical retrace lines were visible.
For this, like many faults it would be a matter of getting in there with the scope, initially looking at the output of the gun amplifiers and if the defect was there tracing through the circuit back to the input, and looking at the blanking voltages too. Some of the CRT blanking circuitry is very close to the very front edge of the pcb, near the bottom of the CRT bulb and that part of the pcb I have seen cracked if the monitor has been dropped, so carefully inspecting the pcb in that area is also helpful.
One of the worst things that could happen with a fault like this, is that it could go away before you find it, so disturb as little as possible before making the tests with the scope.