Hello, there can be multiple causes and solutions to this type of problem you have but my guess is that there is a service not running properly for the screensaver as in THIS
To view services to see if the Human Interface driver is working go to start->Run->type in the box "services.msc" without the double quotes, and then find the service mentioned in the microsoft article.
OR you can use the registry editor to modify your screensaver settings (PLEASE ASK FOR FURTHER HELP OR CLARIFICATION WITH THIS / USE WITH CAUTION IF YOU DONT KNOW EXACTLY WHAT YOU ARE DOING).
SImple example: Go to start->Run-> type in "regedit" and then go to the following registry entry/folder: HKEY_CURRENT_USER\Control Panel\Desktop\ there you can find the screensaver key named:ScreenSaveActive, if set to 0 set it to 1 and vice versa to enable/disable screensaver modify settings. More info HERE