You will find several files in the framework installation path (\%SystemRoot%\Microsoft .NET\Framework\%VersionNumber%\CONFIG\) :
machine.config.xml - the current machine.config file being used.
machine.config.comments - contains a descriptive comments for each key in the machine.config along with it's default or recommended value.
machine.config.default: contains the original machine.config values
To restore the original file just copy the content or copy the default file and rename it.