Please try the following steps in order:
1. Clear your browser cookies
Here's how to clear your cookies on multiple different browsers (a description of what cookies are, are on the first link too)
How to Clear Your Cookies on Chrome: https://support.google.com/chrome/answer/95647
How to Clear Your Cookies on Firefox: http://mzl.la/KEnDua
How to Clear Your Cookies on Safari: http://bit.ly/1LD7nXs
2. If that does not work, try logging in through a different device to see if that works
3. If that's not an option, try a different browser. If you're using either Firefox, Chrome or Safari, try using one that you are not using.
If you still need assistance, please contact our customer support team at firstname.lastname@example.org.