Most of the time this problem occur by cache memory full. At the beginning, I manually remove the files in home/var/cache/ after that the site is working perfectly. But this is not a good approach. Most of the time this problem occur by cache memory full. At the beginning, I manually remove the files in home/var/cache/ after that the site is working perfectly. But this is not a good approach.
This is the way Now I found. (make a Cron job to clear automatically)add a bash script in root folder Magento with this code.

#in root sh rm -rf /root/yoursitepath/var/cache/*rm -rf /root/yoursitepath/var/session/*rm -rf /root/yoursitepath/var/report/*

replace /root/your site path with your site var/cache folder path.

And add a Cron job in Cpanel. like every 12hours like:

* */12 * * * /bin/sh /chroot/home/your site path/html/magento_cache_syrup.sh

Thanks. Happy code.