How can I restart my kombucha liquid?
The SCOBY should act in the presence of fresh sugar, so if you are getting cold tea, then it isn't working, assuming all the conditions of time, temperature and clean vessel are met. 

You can try again, dosing with a little more sugar, but if it isn't turning tea into booch then it is probably dead.

Not sure if you cultured that SCOBY or bought it, so you might need to make a new one with good booch and tea.

Also, I've heard that decaf tea is a no-no, so not sure if that entered the picture?