I've suffered the same problem that i have seen here:
The buttons (ok, cancel) in dialogs disappeared suddenly. In my case, only the buttons of the Parameters dialog disappeared. All the other dialogs were ok.
First i've reinstalled C2 trying to fix it but it remains. Finally i saw these posts and i fixed it with "reset dialogs" in preference settings.
In my case i doubt it has been a driver problem. I have my drivers up to date. and nothing failed or closed before it started happening. I was just introducing some events and previewing, and from one minute to the next the buttons disappear. (I was working with the physics behaviour in that moment, idk if that helps).
I just post this to warn you that this bug is still there.
Link to .capx file (required!):
Internet Explorer: yes
Operating system & service pack:
Windows 8 Pro 64 bit
Construct 2 version:
R117 64 bit