Why is is not possible mark text in my Terminal window anymore?
Sometimes things go wonky in the preferences files and you need to delete them to get things working properly again. Once you restart the app, it will rebuild it's preferences.

Try this:

Quit Terminal completely and then delete this file:
(your home folder)/Library/Preferences/com.apple.Terminal.plist

Hopefully that works for you! If you can't see your library folder, follow this quick guide.