My temporary implant fell off. Can a dentist glue it back on? (Photo)

Are you sure that this is an implant ?  If this is the remainder of the temporary crown then yes it can be remade but not just glued in.  This would require a new abutment that is screwed into the implant and then a new temporary crown. 

However it looks like from the photo that this is a natural tooth that has broken at the gum line.  In this case it would require a root canal and a post and a new crown.  Or an actual implant after remover of the root,. There is a limited amount of natural tooth remaining  so this is a very challenging fix.