Fidel,
What happens is that the pump is able to keep operating although its still gunked up on the inside. When it stopped, it wasn't able to overcome the gunk to restart. Starting is always the most demanding time for a pump. Some large industrial pumps have seperate starter to give them enough power to get started. I would suggest setting up a schedule for routine maintenance of your pumps, especially if you are running a reef tank at relatively high calcium and alkalinity.
Scale will form in areas where there is a large pressure change, such as a pump impellor. Even though it may have been running, I think you could see how it may not have been performing at optimum performance. The scale and gunk in there produces friction on the impellor that slows it down and puts added strain on the motor. IMO, routine maintenance will extend the life expectancy of a pump.
Gary
125 SPS, 75 gal. LPS/softie reef, 9 gal. Nano
