How do you reattach a piece of cement board siding that got blown loose by the wind??

Kelley, gently try and lift up the piece that is above it and nail the fallen off piece pack into place. Then secure the upper one back down. You may need to do some caulking at the ends so give a look there and see what you find.

There are little nails that hold this into place. Both on the bottom and through the top where the nails from the shingle above is nailed. These nails are exposed so nothing hiding. If the tile is still intact. You can purchase aluminum siding nails that are exactly like the ones that have pulled out. Slip the siding back into place, aline the nail holes and replace the nails with new. As Dan's said you will need some caulk to seal the edges of the tile so to blend it back into the wall. Unless you plan on painting soon. Put tiny dots of caulk on the nail heads. This will help secure them from pulling out again. If the shingle has been damaged. You can purchase single cement tiles a most home centers. Some come smooth, others with groves. Still others come with scalloped edges. Purchase one or two that match your homes style. You may need to cut them to fit as far as width. If so a tile saw works well for this. Once fitted take a small mason drill and after fitting where the shingles need to go. Mark the shingle so the holes on the top overlapping shingle will be the same on the new. Then using a small drill, drill holes in the shingle so when fastened with new nails you do not crack the new shingle.