Sir,
This type of error occur when i click edit new item at front end
mootools-core.js:609 GET /component/k2/5-hotels/edit/1016?tmpl=component 500 (Internal Server error)
mootools-core.js:587 GET /component/k2/5-hotels/edit/1016?tmpl=component 500 (Internal Server Error).
but if i edit old item no any error
I am using Joomla 2.5 K2 v2.6.9
please send solution.
Thank You

500 Internal Server Error is a server-side error, so i think there is something wrong server-side... since this is a general server error, i don't think that noone can help you with it without at least a link to the page that generates the error, and credentials to reproduce it. Also, server configuration could be handy, so you need to tell us php version etc

I would suggest disabling any mootools modules/plugins and try again, to start with, so you can point to what is causing this, and report back.

In order to see the real error, which is a server side error as Kogos pointed out correctly you need to enable error reporting and set it to development.
Error reporting can be enabled from your site's global configuration options.