I really don't know what you can do, I can offer a
couple suggestions though.  

How severe is the crease?  Is it just bent or has
it actually been folded and left an line/mark
across?  If it is a bend that has left no marks
you might want to try leaving it in your cel book
or putting it under heavy objects such as books. 
Though it is not good to do this normally to your
cels it should straighten it out eventually.  

If it is very creased leaving a mark there might
be a way for it to be restored by a professional.
 I have never had to do this and do not know which
ones are good but I do know that this has been
discussed here and you might want to try searching
the forum for restore or restoration.

I wouldn't get my hopes up of it ever going back
to normal if it is really creased and has a
line/mark across it though ;(  

I definately would not use any type of heat, you
might end up damaging it more than it is now.  You
will really have to worry about melting and
warping the cel.