Fatal error: Allowed memory size |meta.php error

Hi Everyone,

Please help me resolving this error:
Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 72 bytes) in /home/mywebsite/public_html/wp-includes/meta.php on line 574

*this was only issue in one post type,other post type are working fine.
*site is already running for about two years and just a sudden yesterday that error comes out.

I already did the following troubleshooting steps w/c I got in codex and different forums but still the issue is unresolved:
1. Adjust memory limit in php.ini,wp_admin,htaccess
2. Temporary Create php.ini at wp_admin and wp_include (also in wp_content)
3. contact my host to adjust the memory limit

Thanks in advance…