There is currently a new version of the game, V0.03, if you downloaded it from humble bundle you will need to re-download it (don't forget to save your machine files!) If you downloaded it via steam, you will need to update it via the steam platform.

Once you do this, let me know if the issue is still occurring or not.

Also, if you can post this information, that would be useful as well:

Operating System
System Specs
Game Version
Steps to reproduce bug or what you were doing when the bug occured
Please attach output_log.txt, it is located in Besiege/Besiege_Data