How can I restart the text to speech feature?
have you tried entering these two commands in terminal? 

killall com.apple.speech.speechsynthesisd
killall SpeechSynthesisServer


You can do that in activity monitor, I just like terminal better.
Then wait a bit and try the shortcut again. 
If that doesn't work, try to change your shortcut to something else and then change it back.
Also, maybe corrupted plist file?