Visual Studio hangs on build

Try clearing out all the temporary files in

C:\WINDOWS\Microsoft.NET\Framework\<Framework Version>\Temporary ASP.NET Files

This has solved similar issues for me in the past.

This can also often happen in the event that you have a 3rd party control that is not licensed\registered properly. It may be attempting to display a warning\registration UI that is not making it to focus. We have had this issue on our build servers alot.

I have also run into this problem in the past. I was able to correct it by going to Build -> Clean Solution.