water pressure?

Unfortunatly adam you have not provided enough information for a decent answer to this. I would suggest that it is unlikely due to the heat exchanger as this would not happen suddenly, but over a long period.
Are all of your hot taps running slow (if its just one its down to a tap washer). is the cold mains running slow, is your stop tap fully open.
What make and model of boiler do you have, it may be down to a blocked filter on the inlet.

Richard
August 2010