A tenant once left an iron shaped mark on a practically new carpet. I managed to sort the carpet, which had been melted, by using a pack of disposable razors (20 or more) gently slicing off the blobs at the top of the pile - they have to be quite sharp else the method doesn't really work. The result was generally undetectable although I felt that knowing where to look I could still find the spot. No doubt if the mark is worse than the one I dealt with then this solution won't work.