Can't select external drive to boot

We have been running out mac off an external drive since the internal one failed about a month ago and everything has been going smoothly. We had a power flicker today and I went to power up the computer while holding down the option/alt key. I get to the selection screen and it shows the external drive as available for selection but when I click on it, nothing happens! I've tried double clicking, hitting enter, clicking and then hitting enter... the arrow button on the screen darkens and reacts but then nothing happens and the computer does not boot, it just hangs on that screen. The internal HD no longer shows up on this screen anymore. Please Help!!!