I would start by resetting preferences.
Close Lightroom. Then click the shortcut to launch it, and immediately press and hold Alt + Shift.
This dialog should appear (if it doesn't it might be minimized to the taskbar), click Yes.
If this doesn't fix the problem, try uninstalling and reinstalling (use the CC desktop app to do this).
When you reinstall, the base version, 2015.0 will be installed, and you you will have to update it.
If updating fails, you can download the latest update (2015.10.1, or another version if you prefer) from here:
Edit: The Import and Export buttons are not available in Develop, you should see Copy and Paste in their place.