You probably need to recover grub if the machine is not booting. That should give you a prompt when booting that you can use to select windows to boot into. The link I provided should describe how to do that.
In addition you should run '$ sudo update-grub' on linux if there is no grub prompt when booting, or windows is not listed.
If that fails and you need to get Windows up and running immediately then you can rewrite over the entire disk by looking at How to remove Ubuntu and put Windows back on? (thanks user68186). If you need your data, you can retrieve it by using a live USB stick.
In addition, the answer for How to remove Ubuntu and put Windows back on? describes a program called boot-repair that can fix many boot issues.