@Samssonart
I have also recently encountered this bug unfortunately. After putting in dozens of hours of work into career mode, it's extremely frustrating.
It it helps, I'll describe how it happened.
First, I had about 10 or so saved ships. I didn't and have never turned the game off/quit in the middle of a loading screen (unless the game crashes).
I had just launched a large-ish ship (295 parts) into Orbit, planning on rendezvousing with it. After launch I went back to the Space Center and into the VAB. I launched another, identical ship (295 parts). While loading, the screen froze and the audio messed up (continuously repeating a short sound 20-30 times a second). After 10 or so seconds of this, the game crashed to the Xbox Home screen.
After restarting the game, I noticed all of my quicksaves were gone (with the exception of the Persistent). I then checked the VAB, and noticed that all of my ship saves were gone as well. At this point, the "Return to Space Center, Launch, New Craft, Open Craft, and Save Craft" options weren't greyed out yet. I tried making a small 3 part rocket and clicking Launch. Nothing happened. And now all of the previously mentioned buttons are greyed out. I can't do anything, even exit the VAB.
Restart the game. Load my Career Save. Enter VAB. Return to Space Center. Go back into VAB. All buttons mentioned greyed out.
Very disappointed. I've encountered tons of other bugs but nothing like this.
*EDIT* : After reading some other reports of this issue (and trying to fix it so I can launch crafts again), I haven't been able to do anything that allows me to launch from the VAB. However, I can launch from the Space Hanger. Thought I would include this as it might help with narrowing the scope of this bug.