After installing windows 8.1, it removed my Mac OS X I am able to access the Gray screen but then what do I do to one get OS X Yosemite back and remove the windows partition all together.
MacBook Air (13-inch, Early 2014), OS X Yosemite (10.10.2)
If you have Control Panel -> Bootcamp in Windows, check if you can switch to OS X. If you cannot or get a question mark on a folder, then OS X has been corrupted.
You can either use Internet Recovery - OS X: About OS X Recovery - Apple Support, or OS X: Installing OS X on an external volume - Apple Support and your internal drive can be inspected for possible recovery.
If your Mac has Grey disks, use that to boot from and Disk Utility can be used to look at your partitions.
How do you reinstall OS X and then do you need to manually remove the partion?
