Ubuntu 12.04 LTS on VirtualBox 4.1.18 reverts to last snapshot

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http://askubuntu.com – Somehow, my Ubuntu 12.04 LTS running in VirtualBox 4.1.18 (W7 Host) got stuck.. and I closed the machine. Now when I start up the VM again, it reverts back to my last snapshot which is more than 20 days old. Fortunately I have a quite recent backup of my important data, but still, this is very annoying.. What could the problem be and is there a way to get my data back? (HowTos)