Wish i could help. I'm only familiar with Nintex Automation Cloud and Nintex Office 365 products. 

@Sneh bharti , it seems this was logged in the wrong forum, but, what can you see in the windows error logs?

Edit the application pool and reenter the password to see if perhaps the password was updated and now the application pool needs the new password.

Was this working before?  If it was not working before, perhaps the account needs certain permissions/configurations as per:

https://help.nintex.com/en-US/nintexautomation/icg/5.7/Content/Prepare/SetPermSA-IIS.htm?Highlight=application%20pool