Really I would uninstall reinstall that fixes this most times however
Many fixes for this some are as follows
It has most suggestions
Another 1 posted by a user is
Posted by Gerryhiles
Next time it happens to you try this: check the warning flag in bottom right hand corner and see if it is red. If so then click on it and you'll be given a list and be able to turn on Windows Defender. That'sit.
If issue still persists, please contact technical support chat from this link http://service.mcafee.com/TechSupportHome.aspx?lc=1033&sg=TS and they will remote in for further diagnosis