asp.net: Code Works > Added a reference > Gets an error > Removes reference but error still occurs

I had a working piece of code. I tried adding a button click event but was getting an error. I thought that adding a reference to System.Web.WebPages would fix the problem but then I got a giant error saying:

Could not load file or assembly System.Web.WebPages or one of its dependencies.

The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.IO.FileLoadException: Could not load file or assembly 'System.Web.WebPages' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

So I decide to delete this reference but I'm still getting this error. Why am I still getting this error even though I removed the reference