You may find numerous fixes online to this problem, such as trying to login from different browsers, if on localhost, replace localhost with IP i.e. 127.0.0.1, increase session lifetime, save session data in locale.xml instead of DB etc etc. Here is how I fixed it. I went into the /var/cache folder and cleared it (I mean deleted its contents). I was then able to log into the back-end.
My client told me that she was able to login from her phone browser. Now this seems to have something to do with sessions/cookies. So incase the above doesn’t work for you, also clear the /var/sessions folder. Hope it helps 
