If this started over the weekend it's the by-now-well-known faulty DAT release that's doing it. The easiest way to get back online is to use System Restore to go back a few days (and undo the results of the glitch). Failing that, if you uninstall McAfee you will be able to go back online; download and run MCPR, and then re-install. Although if you get the faulty DAT back again you're back where you started.
There will be a fix out just as soon as the development teams can get it organised. I hope it's soon, and so do a lot of other people.
The faulty DAT has been withdrawn so no chance of getting the bad one back.