Answer:
When you cannot get a computer out of "power save mode", the culprit usually is an empty battery on the motherboard of your computer.
Typically, 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, such as date and time, when the mains power is off. When your computer detects an empty battery, it goes into a low-power mode.
The computer will then display as message, like "Power save mode - press any key on keyboard or move mouse". However, in certain circumstances, doing so does nothing.
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.