VirtualBox: Start tag expected, '<' not found
Check the files of the VM in the VirtualBox VMs folder.
If the .vbox file has size zero, then copy the .vbox-prev file on it.
Restart VirtualBox, it should be repaired.
It seems that the configuration file where the machine settings are stored is screwd up.. The easiest thing to try is to create a new machine whith the same settings as the one not working; but when it asks for creating a new harddrive, select use an existing image and select the image for your virtual machine's harddisk (probably work.vdi)..