Did you copy the entire solution directory to new directory or rename the directory?
There are some hidden files that store visual studio settings and I believe some of the mappings that are established for web projects. Look for a file with the extension "suo" in the solution directory and remove it.
When you try to open the solution again, VS might prompt you to enter the location of the web project. It may find an existing vitual directory with the project name and when it does it will see an existing project file (despite the fact that it's the one you want) and prompt you for a different virtual directory name (usually something like "<projectname>_1"). You might need to instead use the "File->Open->Project From Web..." menu choice to open the project.