Solutions despite not enough description :
1) Install latest version of VB.
2) Stop dbus and try again.
3) There should be enough space in the drive where your virtual box hard drive has been created, check that.
4) Try to create another virtual hard disk and check if your VB-OS is working there fine or not.
5) One amazing solution, I read somewhere.
If you have installed VB on windows. careful, you should not plug in your USB hard disk while starting VB.
Yes you read correctly. Having USB hard drive (Seagate) plugged in made VirtualBox not start correctly.