After upgrading to Yosemite, my Windows 10 partition is inaccessible/gone! How can I fix this?

Pretty straightforward question; I can't access my windows portion anymore. This is kinda a big deal for me... cause I play a lot of games on it. That said, the *space* is still there, but it's been renamed from "Windows" to "DISK0S4" In Disk Utility. It is not viewable upon startup. Please keep in mind the fact that I have very limited computer knowledge when it comes to partitions/partitioning in general. A very close friend of mine did all of the partitioning for me about a year and a half ago. I've since lost contact with him, unfortunately.

I haven't tried anything yet, because quite honestly I'm very lost with what to do, or try. I'm scared I'll do something to royally mess something up!

Laptop specs:

Macbook Air (Mid 2013)

Version: 10.10.5

Processor: 1.7 GHz Intel Core i7

Memory: 8 GB 1600 MHz DDR3

Graphics: Intel HD Graphics 5000 1536 MB