There is currently a bug that causes the game to freeze when played on some machines that have AMD hardware. The team are looking into it, and a bug report has been filed here[bugs.kerbalspaceprogram.com].

The current workaround is to play the game with terrain shaders set to something less than maximum. This unfortunately means not being able to view the improved terrain made available in 1.8.x, however the game will at least work.

If you have any AMD-related issues even after lowering the texture shader level, then please provide any relevant details as a post to the bug report linked above. Sorry for any inconvenience.