Can't login to Magento admin

Question

I have magento installed in a subdirectory. www.domain.com/subdir/magento

This site worked perfectly at one point. I changed nothing, until my client said he could

Answer 1

I was facing the same issue and at the end i found that it was due to full disk space and due to this Magento was not able to create sessions file in the var var/session folder. After cleaning up log files that issue was resolved.