There are three things that may have failed: the power supply, the SATA to USB controller in the drive case, or the drive itself.
I would narrow it down by first testing the voltage at the power adapter; if you do not have a multimeter ask a friend if they can help you. the voltage will be labeled on the power adapter. If that is fine, I would then remove the hard drive from the case and directly connect it inside my PC. If it worked I would purchase a new external drive case and mount the hard drive in that.