Is there a vapor barrier (tar paper) between the sheathing and the metal roofing? Is there any ventalation at the eaves and ridge? As long as the sheathing is still sound you may be able to correct the problem.
There should be ventalation at the eaves and ridge along with a min 1" air space between the sheathing and insulation.
Install ventalation if necessary, Remove and install new insulation with a 1" airspace.