I live in a three story/18 unit condo building. I was notified that the water would be shut off one particular day. Before that day I had less than desirable water pressure but now it is nonexistent. Iwas told that it was probably just gunk built up in the pipes. If that is the case how do I fix it?

"I was told that it was probably just gunk built up in the pipes. If that is the case how do I fix it?"
You shouldn't have to do the work yourself. The other units should also be affected. This makes the problem a common one,(all units affected) and the condo board should pay the tab for flushing out the mains,branches and faucets. Good luck, Tom