How to open the HP 19 inch flat screen monitor

update drivers for that specific monitor

Your computer might have a virus.

Use a flashlight and shine on your screen. Look carefully is you still see the image on the screen. If you do, this is an indication that the monitor is still working, but the backlight has failed. A common solution with backlight-failures is to open the monitor, and look if you see any "swollen" capacitors (http://en.wikipedia.org/wiki/Capacitor_plague). If you see one you should replace it. Only open your monitor if you know what you're doing and have basic knowledge of electronics and soldering.

Thr power switch is a tactile momnetary contact switch setup, when the button is pushed, the contact is closed and open when released. Are you sure the switch is bad? not the circuits inside?
Most common failures in the LCD monitors are bad capacitors (bulging top/seal or leaking) in the power supply, blown fuses; poor solder joints, failed inverter circuits (blown fuse, shorted transistors, shorted/open transformers), bad lamps (poor solder connections or worn out lamps). You will need to open it up and inspect the inside, see example of failed monitors to get some ideas what to look for: http://s807.photobucket.com/home/budm/allalbums
Post back what you see inside so we can guide you further and it will help out other people in the future also.
http://en.wikipedia.org/wiki/Capacitor_plague
http://www.badcaps.net

There are four tabs at the bottom edge of the case tha needed to be release to open up the case.
Most common failures in the LCD monitors are bad capacitors (bulging top/seal or leaking) in the power supply, blown fuses, poor solder joints, failed inverter circuits (blown fuse, shorted transistors, shorted/open transformers), bad lamps (poor solder connections or worn out lamps). You will need to open it up and inspect the inside, see example of failed HP monitors to get some ideas what to look for: http://s807.photobucket.com/home/budm/allalbums
Post back what you see inside so we can guide you further and it will help out other people in the future also.
http://en.wikipedia.org/wiki/Capacitor_plague
http://www.badcaps.net

Most common failures in the LCD monitors are bad capacitors (bulging top/seal or leaking) in the power supply, failed inverter circuits (blown fuse, shorted transistors, shorted/open transformers), bad lamps (poor solder connections or worn out lamps). You will need to open it up and inspect the inside, see example of failed monitors to get some ideas what to look for: http://s807.photobucket.com/home/budm/allalbums
Post back what you see inside so we can guide you further and it will help out other people in the future also.
http://en.wikipedia.org/wiki/Capacitor_plague

http://www.badcaps.net

If you are going to DIY repair, please read below and look at the pictures so you will know what to look for, most LCD monitors have the same problem.

Most common failures in the LCD monitors are bad capacitors (bulging top/seal or leaking) in the power supply, failed inverter circuits (blown fuse, shorted transistors, shorted/open transformers), bad lamps (poor solder connections or worn out lamps). You will need to open it up and inspect the inside, see example of failed monitors to get some ideas what to look for: http://s807.photobucket.com/home/budm/allalbums
Post back what you see inside so we can guide you further and it will help out other people in the future also.
http://en.wikipedia.org/wiki/Capacitor_plague

http://www.badcaps.net

Probably find the screen inverter has failed and needs replacing. Can be a fiddly job as you have to dismantle the monitor so take it to your repair shop if you're not familiar with this type of repair.

The power supply within the monitor has probably expired. HP doesn't do spare parts for their TFT monitors as they are Full Unit Replacement items (if in warranty).

Your logic board need to service.