Yep, be careful when updating NVIDIA drivers. They won't stick to the series you expect, so you could be sitting on 550 and then suddenly get bumped to 580 (even with the nvidia-driver-550 package installed).
I'm now extremely conservative when it comes to upgrading the NVIDIA drivers (mostly using apt-mark hold); and I always make sure that I can roll back before I proceed.
By the way:
- You can download many of the old drivers from Nvidia Graphics Drivers (stable) : “Canonical Kernel Team” team;
- For some details about how to find the specific driver and how to manually install it, try: Hold your NVIDIA drivers if they are stable - #25 by stephematician