If the mysql log – /var/log/mysqld.log (fedora distro) – says:

090728 02:56:59 mysqld started
090728 2:56:59 [ERROR] /usr/libexec/mysqld: Can't find file: './mysql/host.frm' (errno: 13)
090728 2:56:59 [ERROR] /usr/libexec/mysqld: Can't find file: './mysql/host.frm' (errno: 13)
090728 2:56:59 [ERROR] Fatal error: Can't open and lock privilege tables: Can't find file: './mysql/host.frm' (errno: 13)
090728 02:56:59 mysqld ended

Verify the permissions and ownership mysql files (frm, myi, myd) !

chmod 755 for all folder in /var/lib/mysql/
chmod 660 for all files from folders /var/lib/mysql/custom-folder-database/*.*
chown mysql:mysql for all files and folders from /var/lib/mysql/*.* (recursive)