Recently, while testing a web service for code coverage, I encountered a problem where, for some reason, the web service code stopped being covered.

To fix the problem I was experiencing, I had to restore the web.config.backup file.  However, sometimes, recovering the backup file does not fix the problem, and it could become corrupted as well.  The build process modifies the web.config file when executing code coverage tests in web applications.  For this reason, it is imperitive that you keep a safe backup of your web.config file.

Also, keep an eye on the test run warnings and errors.  You can access these from your Test Runs tab by clicking on the Test run errors, warnings link (the name of the link changes when where are errors/warnings).  You will be presented with a list of issues encountered while running the unit tests and covering the identified assemblies of your test run configuration file.