If you still have the piece that broke off then you can simply glue it back on with some superglue.
Use a toothpick to paint the fracture surface, use very very little amount as you don't want it to bead out the sides when you press the pieces back together.
If you don't have the original broken piece then you options become a bit more permanent. Is your PSU modular? If yes then you could superglue the Sata power plug to the hard drive. It would be permanent but should you replace the hard-drive but not the PSU then at least you'd only be down one Sata power cable.
If your PSU is not modular you could still superglue the sata power cable to the hard-drive but now you are basically ensuring the PSU and harddrive both get replaced whenever either one needs replacing first.
Non-permanent solutions include zip-tying the power cable to the drive (gang them together if necessary to get the length you need to go all the way around the drive long-length wise).
Avoid using rubber-bands or non-permanent glues as they will decay and fall apart on you anyways in a matter of months from the heat and vibrations of the rig itself.