You might try starting up in "Safe Mode" (F8 during boot). A default video setting should be loaded allowing viewing. This would eliminate any failure issues.
If you can remember when the change occurred you can select a restore point to a date prior to the change. This should restore display parameters. Caution: doing this will "erase" any updates, software installs, etc. performed after this date.
Or
Right click on display and select "screen resolution" and change your display properties as Ned stated - there should be a (recommended) selection.
Right click on display and select "Personalize" then "display". Try selecting the various options and determine which better suits your viewing.
These actions are for Windows 7 but XP, Vista versions should have similar selections.