Even I faced the same problem. Considering the system requirement have been taken care off and if still you are facing the problem, follow below steps.
Background : You are using Oracle VirtualBox Manager and have selected HortonWorks Sandbox 2.0
1. Click on Settings Tab
2. Click on Display Tab within Settings Tab
3. Check ‘Enable 3D Acceleration’ under Extended Features heading.
4. Click on OK and try staring the VM again.