start the motor and spray carb cleaner/starting fluid around the intake manifolds while it is idleing, and see if the motor revs. I suppose the line from the back of the plenum through the firewall and to the boost gauge could have slipped partically off one of the fittings or it could have a cracked a little and the gauge would be showing the wrong reading.
Was this something that came on slowly or was it all at once?
Is the vaccuum at idle still good?