Meanwhile, there is nothing wrong in adopting idea of solving problems alone but following appropriate procedures is obligatory in getting there with affirmative results. Google on things which are complicated along the way and stick to a step-by-step instruction approach will go a long to help.
Nevertheless, if trying to solve it alone is the intention due to forestalling it on time to continue with a routinely jobs, then follow the steps below are meant for you.
Step 1: Put on the system and enter into the BIOS - most computer Setup key is F2. For others pleases, visit this link to find your system Setup key
Step 2: *Take a closer look at the 'Main information' section to locate the system hard drive message .Probably, it won't be there the reason for the error message of 'Operating System is not found'.
Step 3: Save to default setting, exit and turn off the system
Step 4: Open the CPU and check for Sata or the large signal cable connecting the hard drive to the mobo(Motherboard) if properly seated. Remove it and reinsert it properly.
Step 5: Start the system. If there is no changes put it off again.
Step 6: Check the power cable for power supply by replacing it with any of the extra ones left inside the system for anticipation or for other purposes.
Step 7: Restart the system again to check if that's the cause. If there is no any change after this step follow the next one.
Step 8: Insert the Windows installation disk and wait for it to come up. If there is no effect it means the disk drive is faulty or has malfunctioned, which has to be replaced with a working one.
*Mainly for curiosity and familiarization with your BIOS(Basic Input and Output Setup) area.