Pavilion all-in-one PC black screen

[w_w_s]

Hi, my old pavilion all-in-one PC cannot switch on. The power LED switch just blink once and go off when I press the power button.  When I hold the power switch a little longer, the power LED switch blink at about 5 sec interval. The screen remain blank. However, I can only switch off the PC by switching off the main power instead. 

Please advice what could have caused this issue and how to rectify the issue.

Thank you.

[itsmyname]

@w_w_s -- my guess is that the computer's external power-supply has failed, and must be replaced.

Do you have a Multimeter that can show the Voltage & Amperage being produced by that power-supply,

and to compare those actual values with the values on the label of the power-supply?