Today one of my costumers PC I found the following problem A message on the screen saying that there is no bootable disk.
To troubleshoot I did the fallowing procedure
1) open the case
2)check the hard disk. i cannot sense any vibration of hard disk spinning. and hear small moaning from the disk.
3) check voltages in power cable for hard disk , found the 5 V supply is less than normal
4) evidently there is some problem in 5v regulated supply of SMPS power supply.
5) Replace the SMPS box with new one solved the problem
6) Hard is now spinning.and computer booting without any complaints