Probably not a catastrophe! First, can you twist the fuselage back in shape with the covering in place? If so, you can probably just over twist it a bit and re-iron the covering while holding the reverse twist. This may take another pair of hands. This will allow the covering to reform to a new shape and hold the fuselage straight. You may have to repeat this process several times to get things where you want them to be.
If you can't twist the fuselage to a straight position, try removing one panel of the covering. This should allow the twist. First follow the above directions then add back a new piece of covering where the one was removed.
Absolute worst case, just remove the covering and be more careful when recovering. Many structures are actually held in place and straight by the covering so warps are easily introduced during the covering process. When covering, check, check and recheck during covering. When you do something on one side, follow with the same on the other side so the pulling forces counteract each other. Be thankful you used an iron on covering that is easy to removed with a bit of heat. A real catastrophe is when you have the same problem with silk and dope or an equivalent painted covering.