Run the check
Record whether the monitor's menu appears during a blackout.
Check whether sound and keyboard input continue while the image disappears.
Compare cable, port, and a second source one at a time.
Read the result
A working monitor menu with missing image points first to signal or source.
Limits and next step
A web page cannot report a blackout after its own rendering disappears.
Repeat the check after changing one setting or connection. Keep a record of what changed before deciding whether the device needs service.