Checked my PC

This afternoon, checked on the PC.

Removed printer, scanner, wireless adapter.
Yet same problem occurred.
Conculsion: Printer, Scanner and Wireless Adapter are OK.

Left printer, scanner and wireless adapter unplugged.
Change Keyboard and Mouse.
Yet same problem occured.
Conclusion: Keyboard and Mouse are also OK.

Change back Keyboard and Mouse.
Change Monitor.
Nothing happened.
Conclusion: Original Monitor got problem?

Switch off the system and on again.
Still working well.
Hmm...

Change back Original Monitor.
Nothing happen.
Conclusion: Original Monitor also OK?

Switch off the system and on again.
Still working well.

Plugged back printer, scanner and wireless adapter.

And I used from 3-4pm till now.

Hmm...

So what is wrong or what went wrong?

Guess I'll shut down and try again later with the other monitor. *sigh*