When I try and log in I get this message

Can someone help me with this: I get a message when I log into the CMS. I navigate to web. prompts me to sign in with credentials ( I know my username and password) and I get a message

i havent logged into the site for 3 months. but never had this issue

“sorry the form has expired” I am running 1.7

Please can you try completely clearing your browser cache - this is usually an indication that the page has been cached with an old form token (which is expired)