What should you do if the birth control patch falls off?

Put it back on soon. Don't allow too much time to pass before you have the patch in contact with your skin. Over time medication levels may fall resulting in spotting and potentially contraceptive failure. The adhesive doesn't work well for some women and you may need to use some additional tape to maintain skin contact. Re-read the instructions to be sure you are placing the patch properly on the skin.
Replace it. Place it back on your skin, if it will not adhere then replace with a new patch.