single green flashing indicator light on bottom of MacBook Pro 15"

My MacBook Pro 15" shut down mysteriously and will not power back on (black screen, no sound, no start-up attempt). I plugged in the power cord to make sure it was not a dead battery. When I turned the MacBook over to try removing and then reinstalling the battery, I noticed a single green indicator light (out of the 5 small indicator lights on the actual battery) was flashing. I removed the battery, waited a few minutes, reconnected the battery and tried to power the laptop on again. Again, nothing. However, I did notice that even after removing the battery and reinstalling it, the single green indicator light on the battery remains flashing. Any ideas what has gone wrong and how to fix??? Thanks!