|If it is the hard drive going down... might be useful to try and save all your personal files asap... Copy them to external media - typically DVD; and if possible to an external hard drive too. Check the copies are accessible too on at least one working system if possible. Then put them away safe (and suitably labeled too of course).|
If the system won't boot up at present, the boot with a Linux type of disk - Ubuntu is the usual flavour (ubuntu.com). Download the ISO and burn to a DVD. Boot your computer with that DVD. It will load itself into RAM only, and then your installed HD is available as resource for it; and you can locate and copy your files to DVD that way.
If the system will boot from the Linux DVD then it does tend to suggest it's not the motherboard going down...; but possibly a problem with hard drive (HD)? And presuming the latter - again copy/save your files to external media as above.