I am using Win 7 on Sony vaio laptop (model number VGN-NR11Z/T). Recently I tried to start it, but it shows " no operating system found". I checked the bios and "Hard disk: None". I applied the default settings in BIOS and restart the laptop, but it didnt improve. So I gave up.
After few hours, I again check bios and surprisingly it shows my hard disk. So I start the laptop and was working in it. Suddenly the laptop went on to shutdown automatically without any notification and the same message "no operating system found" was appeared.
Then many times I have seen hard disk listed in bios and can login into my laptop for few minutes. Also many times It didnt show any hard disk in Bios.
You need to determine why the BIOS can't find the hard disk.
Possible reasons are:
1. the hard disk is failing and needs to be replaced.
2. the cable or connector from the computer to hard disk is not making good contact and needs to be cleaned or replaced.
3. the laptop motherboard is failing.
This diagnosis will probably need to be made by a qualified PC Tech.
You may be facing some connectivity issues. Is this a new hard drive or you have been using it some time? If the laptop is under warranty, contact the seller and ask for guidance.
If the warranty has expired, I would open the laptop and check if the hard drive is properly placed. Remove it and reconnect it back. I strongly recommend you to backup all data the next time you are able to boot. It is really hard to tell you if it is a HDD problem or a laptop problem and, as xpcman said, you may need to consult a technician.