Did you recently update your motherboard's BIOS? If so you need to set your configurations back to the way it was.
Firstly start your computer and go into the BIOS. Then look for the option which states "SATA Controller," "SATA Selection Mode" or anything similar. Set that to AHCI, save your settings and restart your computer. Hopefully you should be able to boot back into Windows.
If your selection is already on AHCI, try setting it on IDE.
If that did not solve your problem, could you please provide more details? Since when did your system not boot into Windows? Did you recently update your BIOS, install any software, or make any hardware changes? Also does your computer reboot whilst in the BIOS? If not, that could indicate a...