Your Fedora machine does boot, it is just that the X Server that you are used to does not start. At the black screen with the flashing underscore, hit CTRL-ALT-2 or CTRL-ALT-3 and login as root at the console. You now have your machine in a usable state.
You can then downgrade the x11-xorg server to a 1.17 version.
If you are not familiar with running this type of thing, then you probably should downgrade to F22 immediately if you can at all. This is not super trivial to fix and will require some work and research on your behalf.
If you are confident at the command line, then I highly suggest that you start looking into the process as it only took me a short time to downgrade xorg-x11 to 1.17 (from Fedora 22 repos) and get up and running again with a Fedora 23 installation (everything except xorg-x11 was standard F23) using the latest NVIDIA driver. This will be extremely valuable to you later, trust me.
As a side note, I am having a few other issues with F23 that are a bit painful with respect to specific applications not playing nicely, this however has nothing to do with the NVIDIA driver, but it is why I am not recommending anyone specifically upgrade from F22 to F23 right now.
Also, you will note that F23 release has slipped another week and this issue (XORG-X11 v1.18 causing issues with NVIDIA) is not one of the reasons for the slip. This problem is not going away and until an NVIDIA driver is released with ABI support for V1.18 then it will be a problem in F23.