Hi Sharon - Snap may not work with some programs that have custom window behaviors. Since you said Snap worked beautifully last week but is not working now, I wonder you have made any changes to your computer in the past week that are now effecting Snap. What two windows are you trying to Snap?
Just to make sure, are you following these Snap instructions?http://windows.microsoft.com/en-US/windows7/Arrange-windows-side-by-side-on-the-desktop-using-Snap
Microsoft Windows Outreach