You have ice at the Tee's under the shower. Leave the water running (watch your holding tanks to make sure they don't overfill) and it will take a bit of time, but the ice inside the Tee's will melt and things will start working properly. If you have a small electric heater, you might want to put it in front of the cupboards on the back wall to help. Otherwise, it's just wait for the heat to get to the pipes and thaw them.
Remember, it takes 3 or 4 hours to thaw a chicken breast, it takes as long or longer to thaw a "partially hidden pipe"..... Patience is a virtue
