If the “wheel just keeps spinning” then it means the browser thinks it is still loading something (from somewhere). If you open your browser developer tools -> network tab (and reload the page) it should eventually show “Loaded” at the bottom, in red, ex:
For the waiting-on-resources, you might be able to see one in the “network” list that is attempting to load but not finishing.
The other links in the other answers were useful as well (one of them said, in my case “browser ran out of memory” or “unable to connect to server” all indications of my problem). Another possibly useful link is this.
In my case it appeared to have been a conflict between prototype.js #inspect and the google (gapi) plusone button, causing some kind of infinite loop (?) that made it eventually run out of memory. Updating prototype seemed to “fix” it.