Run the check
Open the monitor's own menu and confirm the selected input.
Check source power and reseat one cable at both ends.
Try a known-working input or source, then return to the original.
Read the result
A working menu with no image from one source narrows the external path.
Limits and next step
An unavailable display cannot be tested by a page on that display.
Repeat the check after changing one setting or connection. Keep a record of what changed before deciding whether the device needs service.