The standard power settings for Windows 7 are to put computers to sleep after 30 minutes of inactivity. Normally this isn’t a problem but some computers refuse to let the user wake them up again after they have fallen asleep.
A workaround is to change the power settings and stop the computer falling asleep in the first place. To do this click on the “Start” button in the bottom left, then click on “Control Panel”. Then click on “Power Options” followed by clicking on “Change when the computer sleeps”. Select “Never” from the drop down list labelled “Put the computer to sleep” and finally click on “Save Changes”.