I assume you are talking about the home product so I moved this to VirusScan.
With so little information it's difficult to advise but, make sure your operating system is totally up to date, including IE, even if you don't use it, as McAfee does.
Right-click the taskbar icon and select 'Check for Updates' and see if that helps.
Run the Virtual Technician to see if it finds problems and fixes them automatically: http://mvt.mcafee.com/
If that fails try Technical Support (Monday to Friday in your area), it's free of charge by phone or online chat and linked under Useful Links at the top of this page.
(If you see en email option I don't recommend you use that as it is extremely slow).
Restart your PC and wait for 3 to 5 mins. Try Opening McAfee by double clicking on M shield icon on the Desktop. See if McAfee Opens and Real time Scanning is turned ON. If not post back the version details of McAfee by clicking on About link located at the bottom right of McAfee User Interface.