This is usually caused by changes in a system CMOS area. When the message "Operating system Not Found" is received at the bootup stage of your system, at the point when the Windows welcome screen is about to come up. When you look you discover a disconnected chart with a directive function keys instructing you to depress F1 to restart and when you do it returns back to the same place as if no Operating system (like a system bought new from the shop without Operating system installed) has been installed before. And, an arrow key pointing up to another chart of the missing link is continually appearing and disappearing. That problem is mainly coming from your CMOS area. The Battery may be dead, it could be that your system files are corrupted and altogether your BIOS could not locate the Hard disk or just that your system CMOS battery is not contacting well yet,etc. All of these have to really do much with your CMOS.
If you are experiencing such trouble with your system, the following steps are the quickest ways out.
(a) Opening the system to rectify the problem
Step 1: Turn off your system by switching it off from the main.
Step 2: Get a screw driver handy at your closest disposal.
Step 3: Unscrew the bolts of your system casing and look very carefully. Please don`t touch anything yet until you write down these instructional step by step on a piece of paper for easy follow-up.
Step 4: Look around inside of your system you will see a spot usually circular, like a big effervescence table on the MOBO(Motherboard), that`s your BIOS(Basic Input/Outpout Setup)/CMOS (Complementary Metal Oxide Semiconductor) .
Step 5: Very close by is a pin generally called jumper. If you pulled it up carefully your will find out it`s two pointed pins. Turn it horizontally like rotating it sideways and put it back into the two pointed pins.
Step 6: Then pull out the battery from the CMOS carefully so that you don't damage the contact pole before release. As you can see on the image below.
Step 7: Check for rust and scrape it up. While replacing back the battery, please be cautious of the polarity (the part of the battery with inscription should be up), polarization may cause the battery to explode or your BIOS CMOS setup will not come up.
Step 8: Screw back the nuts in their appropriate places or slot back if that is the case with you computer casing.
Step 9: Connect your system and start it up as usual by depressing the power botton. The system will immediatly take you to the BIOS setup make changes and save it. The system will detect your Hard disk this time.
(b) The system did not detect the Hard disk
Step 1: You will have to replace the battery with a new one and follow the previous instruction from Step 8.