It usually because the system cannot find the hard drive on which the os is installed. This can be changed via the BIOS during startup by pressing either F1,F12,DEL or other keys depending on your system. You can see which key to press when it boots...It tells you for o say 2 seconds which need is used to go to bios. Once you go into BIOS, just change the boot order. It lists your boot sequence...like cd rom, floppy drive, master drive, slave drive and there you should see your hard disk drive.
Make sure you can see it first, If you do then change it, and save changes before you exit. a
)If you cannot see it, remove the power cable, Press the reset button for few seconds(this will drain the rest of the power in the system), open the side panel, remove the cable from your HDD(both power and scsi) and then put them back and make sure it fits snuggly. b
)Remove the memory chip by unhinging the white tabs, wait for a few seconds and put them back and ensure that it fits properly and you hear 2 clicks(one from each side)
Close the side panel, plug the power back on and turn on your machine.
This time let the system do its POST(Power on self test) completely and see if it works. If it still gives os not found, Go back into the bios and change the boot order.
If you do see the hard disk this time around and it works, the reason was that
The system magically reset itself. It happens 1 in 100 computers. By removing the ram and plugging it back on...you manually reset it. Just like how we go banana's sometimes and someone has to calm you down by hugging, patting and removing us from that scene. Senario 2
There is no OS installed on your computer period.....hahaha.