Element UpdatePanel is not a known element. This can occur if there is a compilation error in the Web site

Element 'ScriptManager' is not a known element. This can occur if there is a compilation error in the Web site. 

 Element 'UpdatePanel' is not a known element. This can occur if there is a compilation error in the Web site.

tried many things checked web config file reinstalled ajaxtoolkit

but nothing is working

getting error in the whole project along with this getting error for other asp,net controls also

Element 'TextBox' is not a known element. This can occur if there is a compilation error in the Web site.