Fix Error namespace name 'Linq' does not exist in the namespace 'System'

Fix Error The type or namespace name 'Linq' does not exist in the namespace 'System' 

I had an issue with System.Linq not being recognized. The way I solved it was to change the targeted framework of my website from 4.0 to to 3.5 and then switch back to the original targeted framework (in my case 4.0)

1. Hit Shift+F4 in Visual Studio.
2. Change the Target Framework from .Net Framework 4 to .Net Framework 3.5
3. Confirm with OK
4. Repeat this process in reverse so, again hit Shift+F4
5. Change it back from .Net Framework 3.5 to .Net Framework 4

Hope this helps