Not allowing me to log into my account

When I opened minecraft earlier it said play offline were it usually just says play. So I logged out of the account opened the launcher again and tried to log in. However it would not allow me because it says that my account is not online. Underneath it says (SunCertPathBuilderException: unable to find valid certification path to request target). What can I do to put my account back online so I can play Minecraft.
Thanks, Nick