The two likely scenarios that I can think of are:
#1 - Windows is corrupted somehow. You need repair Windows. If the laptop shipped with the Windows 7 installation DVD then use that to attempt a repair; it may not be possible. If it did not ship with a DVD, then the hard drive may contain an image or Windows 7 in a hidden partition on the hard drive (a very common practice). Refer to your manual on how to reinstall Windows.
#2 - The hard drive has failed and need to be replaced. If you do not have the Windows 7 install DVD or Windows 7 is in the hidden partition of the failed hard drive, then you need need to purchase a new hard drive and buy Window 7.
If the Windows 7 product key is legible on the underside of the laptop, then you can use it to download a copy of Windows 7, burn it to a DVD and use that to install the OS.
http/www.microsoft.com/en-us/software-recovery