What you should have first is the win7 recovery disc and have it inside a CD/DVD or flash drive(use YUMI)
after booting the disc, go to the command prompt and type the following:
This part makes sure that "access denied" won't show up when restoring mbr.
Diskpart
list vol
exit
This part is the actual input to restore mbr. Again, "D" changes depending on your WINDOWS installation
bcdboot D:\windows /s D:
bootsect /nt60 D: /mbr
OR trie
bootsect /nt60 all
bootrec /fixmbr
Then you're done!