How can I fix my Briggs & Statton mower blade w/o buying a new one?

How can I fix my lawn mowers blade without having to buy a new one? I mean the blade is still in one whole piece and still connected to the lawn mower. It's just that I hit a stump that was really low to the ground and very well hid by the tall grass. Anyway the blade on one end seems to come down 1 inch maybe a little more from where it's suppose to be. The type of lawn mower I have is Briggs & Stratton w/ a Rally 20" blade on it. I believe that one end that hit the stump can be bent back, but when I tried to loosen the nut that holds the blade on it wouldn't come free. Now if I can figure out how to get nut off and that will get the blade off. Then I will be able the information that you will provide for me. Another reason I don't want to have to buy a new mower or blade is because this mower belongs to my landlord, and he would be very upset with me if he found out about this.
Thank you for taking the time to help me!