Out of the blue, with no changes to any files for a long time, our 1.6.7 installation of Expression Engine no longer shows the back end (system/index.php). It simply shows a blank page. I check the files on the server, and their contents are all in tact and unchanged. The front-end content is all in-tact without errors.
The only way I can get in is by using /admin.php from the utilities folder. Nothing looks abnormal in the backend.
Any idea why this would have happened out of the blue?