My colleague was struggling to access admin url. We looked for the issues but couldn't find any. After going through few forums we found this:
In phpMyAdmin find core_config_data and search for below and delete if they have words like admin, manager or your Custom Admin Name.
admin/url/use_custom admin/url/custom web/secure/base_url web/unsecure/base_url
Then, in /public_html/var/ delete cache folder or it's all sub-folders. It's safe to delete cache folder as magento creates new cache folder if it doesn't find one that's already existing.
We saw, so many people said it's working. But in our case it wasn't working. Later we found that admin was changed to manager in local.xml config file by some other staff.
In /app/etc/local.xml and look for <![CDATA[admin]]> or whatever path you have set and update it as your need. we revert back to admin in our case.
After, fixing above file, we were able to access admin again!