You've probably used a third party control that can't be found in your updated project. Try looking under "References" in the project tree in the Solution Explorer. One of the references will probably be marked with a yellow warning triangle, like
THIS[
^], .
That's because a reference is broken. If you click the missing reference, you can see in the property explorer what file it is it can't find.
You need to remove the broken reference and add a new one with the correct file path by right clicking the project node and selecting "Add reference".
After that, the project should build as usual.
Also,
THIS[
^] might be of help to you!
Submit an article or tip