revert to last successful build

Oh, painful.  Are you using source safe?  If so, you can roll back to the prior version of the application using source safe.  If you have backed up your application but have made a lot of changes to it (and don't have source safe) you can use a free tool such as examdiff to compare updated files and find all of the differences between the two versions (and save yourself from rewriting parts of it from scratch), lastly, if you have not closed visual studio, you can control-z your changes out of the failed version and resave the application.