As I have now quickly skimmed the e-book on Git, I may be able to help. I am guessing you follwed Dan’s instructions to get the Blinker_test version of OSPi.
If you go to the OSPi directory (or whichever directory you have as your working directory), and type git branch it should show you two branches – Blinker_test and master. Blinker_test should have an asterisk next to it. If this is so, then if you now type git checkout master you should be back to your original configuration.
Before you do it, (and if you are running the Blinker_test branch) could you go to the OSPi directory and type
sudo service ospi stop
sudo python ospi.py
Then you should see the console messages. Fire up your web browser and try to get the Internal Server Error message again. You will probably see a python error message on the screen. That would be helpful information to see why your system got this error (I am running the Blinker_test branch currently with no issues!)