Comment 56 for bug 1889509

> 1. How can I boot the system from within the Grub rescue prompt? I don‘t have a live System and can‘t get hands on one for several days.

You can't. The part of grub that loaded from the MBR (and printed 'grub rescue>') is incompatible with stage 2 of grub at /boot/grub/. You either need to upgrade the MBR part ('grub-reinstall /dev/...') or downgrade /boot/grub/ somehow (perhaps copy off another system?). Either way you need the install media.

> 2. I did reboot my computer right now, and minutes before I did a full upgrade. From the above conversation one can see that the bug is known for almost a week. So why is there no emergency patch like a package that goes back to the last functional version and does a grub reinstall?

The bug (#1889556) has been fixed. Did you 'apt-get update' and 'apt-get upgrade' prior to rebooting? If so, 'apt-get upgrade' would terminated early with an exit code, which is as much of a hint as dpkg can give you that grub is hosed, and needs fixing ('dpkg-reconfigure grub-pc') prior to reboot.