Older posts problem.

Try:
– switching to the Twenty Eleven theme by renaming your current theme’s folder inside wp-content/themes and adding “-old” to the end of the folder name using FTP or whatever file management application your host provides.

– resetting the plugins folder by FTP or phpMyAdmin.

You mean i should change the theme?

Yes…

but actually i like this design, So i don’t want to change it. Any other way to solve this problem?

Do you want to locate the source of the problem?. If yes then you need to carry out the very basic troubleshooting steps.