I installed proxmox on my boot and:

Yeah, it looks like you will have to reinstall Windows. It seems like you installed it on the disk on which Windows was originally on, since it doesn't seem to be available as boot option anymore.

Did you have a second drive for your Data? For example, did you have a D: drive in Windows?
 
I see. Well, you installed it on top of Windows, most definitely. There's nothing you can do, unfortunately. You will have to reinstall Windows again. It's best if you let a friend help you with that.