Permissions did not change by themselves, someone (or something) changed it. That could happen multiple ways:
- PHP Scripts changing permissions
- User changing permissions via FTP (chmod)
The thing is, to change permissions for a file or folder, you need to have write permission on that folder. This allows you to remove permission from yourself (which happened in your case), which sort of locks you out from accessing the files.
To be able to reset the permissions, you need to login as root and set them to be user-writable and group-readable (or group-writable if needed):
$ sudo chown -R u+rw,g+rx path/to/directory
If you do not have root access on your web-server nor any other way to configure reset permissions (like cPanel, but even that might not work), then you need to contact your hosting provider to do it for you.