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.