Windows 10 will not boot anymore
I can help you if you've a live USB linux distribution, ex. Ubuntu.    


Boot into Ubuntu live USB by pressing alt when upon
restarting and choose EFI Boot.    
Choose to Try Ubuntu.
Open Terminal by ctrl + alt + T.
Install efibootmgr by entering sudo apt-get install efibootmgr.
After installing it, check bootOrder and boot values by entering sudo efibootmgr.
Usually you OS X boot value is 0x80, please get your Windows value.
You can change booting order by sudo efibootmgr -o &lt;comma separated boot values&gt;.
Adjust your bootOrder assuming your Windows boot values is 0x01 then you should enter sudo efibootmgr -o 1,80 &lt;- Windows first. So that your Mac should try booting to Windows first as you mention that you need an urgent solution.
Reboot to your live USB again and readjust the bootOrder by letting Mac boot into OS X first by sudo efibootmgr -o 80,1 &lt;- OS X first.
Don't use any 3rd party bootloaders as you can easily adjust everything manually.


Advice:
Format your shared partition as ExFAT =)