Like many people I've been suffering from not being able to access my account since the potential hack of the Lastpass site.
I finally managed to get it working by following these instructions step by step.
1) Turn off Modem and Router
2) Turn off PC
3) Reboot PC but leave the Modem and Router off
4) Load up your browser
5) Clear both Cookies and Cache on the Browser (very important)
6) Remove the Lastpass Extension from your browser
7) Close your browser
8) Start your Modem and Router
9) Once connected load your Browser
10) Re-install the Lastpass Extension
11) Log back on using your original password before the hack
12) If that does not work and you changed your password after the hack then try the new password.
If you are still not working now try to log in via https://lastpass.com/?ac=1
Failing that email their support support@lastpass.com