Missing Site.xsd

Hello,

Does anyone got this error before and how can you fixed it?  Thanks for any advice!

Geocortex Essentials Application Error

The schema referenced by the document //.../Geocortex Essentials/Default/Web ADF Elements/Framework/Site.xsd cannot be found.

BTW, We're running AGS 10 SP1 with Geocortex Essentials 3.5.1, our map services are working as normal until yesterday.  I don't know why and how the Site.xsd is missing.  The Site.xml is still there. How can I recovery the Site.xsd and prevent it to missing again????

Thank you so much.