Asp.net Validation not working for all project

Hi, i'm researching about for the days but unable to find  a solution, suddenly my web application's validation stopped working, All was perfect some days ago, but now i checked a form its validation not working i go through the rest of application but all validation not working... 

Can anybody guide me in this regard, why this happend have any dependency remove mistakely or something else?

Thankyou