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Hello and welcome to Just Answer. My name is David.
What model is the computer?
Moving the keyboard and mouse should take the computer out of power save mode. If that does not work and if the computer is a laptop, close the lid and wait a few seconds and then open the lid. If that does not resolve the issue then unplug all peripherals and then try again to move the mouse and keyboard.
If none of the above resolve the issue then you will need to reboot the computer to get out of power save mode. I would then recommend disabling the power save mode for now to avoid the issue happening again (depending on the operating system version you can access the power save options by clicking Start and then Control Panel and then Power Options) and then updating hardware drivers (especially new hardware) so that power save mode is properly supported by hardware. Most of the driver updates will be available from the Emachines support web site at the link below: