iPhone application error

Hi!
I am developing an iPhone application. I am 40% done with it. Suddenly it is

throwing an error. I tried going back to the stage where it was running fine in

the simulator, but now the stage at which it worked is not working as well.

Here is the error message:
"_UI ApplicationMain" referenced from:
_ mainin main.o

".objc class_nameUIViewController" referenced from;
.objc class_namecontactListViewController.in

LoLiVi.o

Symbol(s) not found
Collect 2: id returned 1 exit Status

I tried uninstalling the SDK and reinstalling it again. But it still does not

work!

I am stuck and any thoughts would be highly appreciated.