I am terribly sorry to inform you that I have just lost a tremendous amount of the editing I had completed on your paper due to a computer crash. I am going to have to re-do the missing portion. I will try to finish it by the deadline you selected, but there is a possibility that I might have to return it to you an hour later than I was supposed to. 
Please accept my apologies, and I hope that this does not cause any problems for you. I value you as a client and enjoy editing your work. I will try hard to make sure that nothing like this happens in the future.