Q:

Project build failed in xcode

Only for a specific project xcode showing project build failed without any error.
Which was working fine till yesterday. 
Without any change it is not working today.
What is the issue?
What can I do?

A:

In your Xcode navigator, click on the Show the Log Navigator tab.

This tab will contain the build logs for each time your build/compile the application. When an error occurs, this will show you details. There is an expand button to check the details causing the error.  Check the steps until it fails, see if anything fishy is happening during the build process. Usually in error cases the build process will freeze/quit at a particular source file.

Usually I have seen that restarting Xcode would clear its error cache and any stray errors will go away. Also as a quick check create a new sample app and try building it. If you get errors building this app as well, I'd suggest you reinstall the Xcode. If this builds properly, there is definitely issue with your project files/code. In that case you need to add more details in your question.
Hope that helps!

