It looks like the nails have let go. The caulking on the right is probably what is keeping it from falling altogether. If it was shingles you could replace that one, nail through the ovelapping shingle and caulk to seal. I have no idea how you would fix tiles.
Another thought might be to rip it off the rest of the way then, assuming it’s in good shape, use a construction adhesive (gorilla glue, liquid nails etc…) to glue it back in place. I wouldn’t do the whole house with glue, but I don’t see a problem with a single piece here and there. Make sure to recaulk the right edge though.
BTW, this is just amatuer advice to be take with a grain of salt.