Installing new 7

Hi guys.I have a problem.my sister buy a new desktop unit with windows 7 OS 2 weeks ago.For some reason yesterday her system cant load anymore the OS prompting "pleas boot to proper drive or load your system disk then restart again."So i tried to reset the cmos and bios setting to their system default but unluckily the same prompt appeared again.i tried also to reformat the hd but upon entering the option to choose if i want to upgrade or customized installation i choosed customized to install a new copy of windows but the hd wont appear.my sister loast the warranty so she cant just put it back to where she sold it.pls anyone can help me guys.what should i do?the hd wont appear?

Check your HD.
1) Boot DVD, & Press Shift+F10 (for command prompt)
2) Boot DVD, & Command Prompt at Startup (for command prompt)
Open an command prompt.
In the command prompt, type chkdsk C: f/x/r/v and press Enter.

Disk Check
Check Disk - chkdsk - Vista Forums

Check Disk (chkdsk) - Read Event Viewer Log

Check the hard drive with the manufacturer's diagnostic tools.
Hard Drive Diagnostics Tools and Utilities (Storage) - TACKtech Corp.

When in the BIOS is this hard drive even listed there?
If not then check all the cabling to it, make sure all is fine (while no power going to it) try bios screen again, if not there, likely cabling failure, motherboard controller or the inexpensive drive electronics have failed, hard drives are cheap, get another one after testing this one on your own system. (hands you a phillips screwdriver)

Even if you get another one, I'd still try to return the hard drive to the OEM for free replacement, 2nd hard drive is always wise for virtual memory. (look this up if first you have heard of it, I don't need to go off on another tangent, )
Hard drive itself has a born on sticker, check versus the hard drive manufacturer's warranty, forget the store's.
(Might have to disconnect and remove from the case to see it)

If that hard drive is jumpered as master, I'd even try switching to cable select and rebooting again.

thank you guys..i will try one by one all of your advised.