With such sort of question always supply:
-Your Operating System (+version)
-Sweet Home 3D version + (32 or 64bit version)
-Graphics card
-Which installer/download you used

If you installed a 32bit version replace it with a 64bit version.
Install the latest version of Sweet Home 3D.
If you have and AMD graphics processor it's possibly a driver problem. Update to the latest driver.
If you are on a Linux system try to start with the SweetHome3D-Java3D-1_5_2 script instead of the default SweetHome3D script. You can find them in the directory where Sweet Home 3D is installed.