I would either use a Windows installation/recovery disc or USB to try and repair the MBR.
An easy way to determine if BIOS still sees your disk is to go into it (usually press F2, F10, F12 or Delete when the computer turns on) and go to the boot order/priority section. If a hard drive is listed with a model or serial number, then your hard drive is correctly installed and that means the issues is your MBR, which can be fixed easily with a Windows installation disc.
I will highly recommend an additional diagnosis, cleanup, defrag, update, and getting a Windows repair USB stick for safety against it happening again.
