If you have the Windows product key (Windows 7 keys are normally underneath the laptop) then all you need is to download the OS from Microsoft, burn it to DVD and reinstall the system. Otherwise ask for a disc from your local computer place and reinstall it like that using the product key underneath your laptop.
However, if you're saying that it shows "No Hard Drive Detected" on diagnostics, then either the drive is disconnected or there is something wrong with the HDD board. Open the BIOS, it should show the HDD name there. If not, then the drive may need to be replaced. If you can see the HDD name, then chances are the bootloader of the OS is missing so you'll need to run a repair of the OS. Otherwise, remove the drive from the laptop and see if it works in an external enclosure.. If it does, then you can back up everything (if readable) before reinstalling the system.
15 years in the IT Industry