The easiest thing to do is to let the smoothing disk dry out completely, then use a spray adhesive to re-attach the magnet to the disk.
Usually a magnet will start coming off only around the edges of a disk. Just spray the adhesive on both the magnet and the felt underside of the disk, roll the magnet back onto the disk carefully to avoid trapping any bubbles under the magnet and then place something flat and heavy onto the disk overnight.
If you need a good spray adhesive, 3M's Super 77 adhesive does a fantastic job. If you are uncomfortable doing this yourself, just send the disk back to us and we'll re-apply it for you.