This has happened to me many times, and some time ago I found that manually reinstalling Parallels tools after doing a an update in Ubuntu with the Intel x.org drivers resolves the issue. When these drivers are installed Ubuntu has a blank screen without the Unity launch panel. Use the following steps: Use snapshots of the Ubuntu VM instance. This is your backup in case the driver is installed. Look at what is being installed when executing the Software Update app. For safety's sake, if there is anything that is related to x.org I reinstall Parallels Tools BEFORE shutting down the VM. - Erik N.
