It's your cookies, duder.
Adblockers, Antivirus, Anonymous Browsing, etc. are notorious for breaking site logins. See if your Norton, McAffee,
etc. aren't blocking cookies. Good security programs always run in the background, so closing the icon doesn't mean they are shut down (Avast definitely does this). Try a different browser or in anonymous mode to help pinpoint the issue.
Just do it. We have no lives.
If all else fails click this support link email us and describe your problem. Please include your Browser, Operating System, etc. and be as specific as possible so we can nail it on the first try.