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

Did you erase the session storage in var directory?

In my case, when I was playing with autorization for multistore on subdomains (changed path and domain for cookie as you did), this method helped me to drop the "bad" cookie and sucessfuly logined in admin:

In apppath/var/session directory I've made command in shell (be careful with path, this could delete all the files in the directory)

rm -rf /path/to/magento/var/session/*

And then just clean the cookie for domain in browser.