It could be that it is not the monitor which is in "power save mode", but the computer itself.
When you cannot get a computer out of "power save mode" by any other means, typically the cause is that the button-cell-type battery of your computer, which is located on the motherboard, is empty.
Usually, this happens on a computer which is a few years old, since its batteries will have run out after such a long time. (The motherboard battery is used to store various settings when the computer power is off, such as date and time. When your computer detects this battery is empty, it goes into a low-power mode).
The solution is to replace the button-cell battery on the motherboard of your computer:
- open the case of your computer;
- visually locate the motherboard (the main electronic circuit board) of your computer;
- visually scan the motherboard for a button-cell battery (it looks like a digital watch battery, but a bit larger);
- remove this motherboard battery from your computer;
- restart your computer: it should now power up normally (you may get some warning messages during the start-up sequence, but ignore them);
- set the date and time of your computer using the "Date and Time" control panel;
- you can now use your computer normally, but you will lose date and time settings when you disconnect the computer power cord, since there is no motherboard battery present to store them;
- take the motherboard battery to an electronics store and get a replacement;
install the new battery in your computer;
- restart your computer, and set the date and time again; the settings will now be saved using motherboard battery power.