Question: I have a Visual Studio 2010 solution with a few projects. I can successfully build with Build -> Build Solution or Build -> Build MyCompany.MyProduct.MyProject. After building I press CTRL+F5 and successfully run the start up project. This works as expected.

After I change a source file, save it and then again build (with either Build -> Build Solution or Build -> Build MyCompany.MyProduct.MyProject) it builds successfully. I am still able to press CTRL+F5 and run the startup project. This again works as expected.

The problem is in the following scenario:
After I change a source file, save it and directly press CTRL+F5 then Visual Studio 2010 says BUILD FAILED and there are no errors. And it doesn't start my startup project (of course).

Am I missing something?

Answer: 
Try repeating the same steps by changing the location where you compile/run. Make sure your new location has less folders to traverse when compared to your old location. A lengthy path to the source code had resulted in a similar issue to me and I overcame it by choosing a shorter path.
In case some other folks are experiencing this issue (I had tried deleting the SUO file first but this didn't work for me):

I solved it by creating a blank new Project in my solution and importing all source files into the new project. Likely the problem sat in project file.