Unable to log into front end

Hi guys,

I appear to have lost my ability to log into the front end of my blog. When I input my username and password, it's returning an "invalid token" error. I am logging into the admin end no problem, but I need to preview my posts before they go live, which I can't do if I can't log into the front end.

This appears to be an issue isolated to my iMac, as I'm logging in no problem on the iPad.

Any ideas?

Thanks!

Sue