Sounds like the screensaver is kicking in. To change the settings, you need to go into the system settings. I'm not a cinnamon user though, so I don't know what categories of options you have in the settings for cinnamon. For most desktops it's usually under something like power-management or screensaver. Once you've found the appropriate section in the options, you can either disable the screensaver, or increase the amount of idle time before the screensaver kicks in.
