Symptom Analysis: Why Did Ubuntu Suddenly Drop to “Low Resolution”?

When your Ubuntu system boots up and the screen resolution has suddenly dropped significantly (e.g., from 4K/1080p down to 800×600), there is a nearly 90% chance it’s related to the graphics driver.

In Linux systems, the display driver (especially with NVIDIA cards) relies heavily on the Linux Kernel modules. When a major kernel update occurs, the old driver modules may fail to compile or load correctly on the new kernel, causing the system to automatically revert to the most basic display mode. The situation you encountered is a compound problem:

• Problem A: Low Resolution (Kernel and Driver Conflict).
• Problem B: failed to fetch error (Network or Package Repository Configuration Issue).

Solution Process: Isolate Variables, Revert and Repair (Core Steps)

The key to solving this problem lies in: isolating the kernel variable.

Step 1: Enter the GRUB Boot Menu and Select a Stable Kernel (Crucial)

1. During startup, enter the GRUB Boot Menu.
2. Select the second option: Advanced options for Ubuntu.
3. You will see a list of kernel versions. Choose the version immediately preceding the newest one to boot.
   • For example: If the newest is 6.14.0-28-generic, choose 6.14.0-27-generic.

Step 2: Verify Resolution and Driver Status

If the older kernel boots successfully, your desktop resolution should immediately return to normal. This confirms that the root cause of Problem A was a kernel update incompatible with the driver.

Step 3: Fix Package Repository and Errors Under the Stable Kernel (Solving failed to fetch)

In the stable system environment (older kernel), re-run the full system update:

sudo apt update
sudo apt upgrade

This action repairs and fully downloads all necessary packages for the latest kernel, while also fixing any potentially corrupted package lists. After execution, ensure no severe failed to fetch errors appear.

Step 4: Restart the System

After the repair is complete, restart the system directly:

reboot

The system will automatically select the newest and complete kernel to boot. At this point, the resolution should be normal, and running sudo apt update again should not produce the failed to fetch error.

Summary

It was lucky that the problem was solved by chance. After Googling, it seemed that fixing the failed to fetch error wasn’t that simple.

This experience perfectly illustrates the basic troubleshooting process for Linux systems: First, solve the core hardware/driver issue (low resolution), then address the secondary network/software problem (failed to fetch). By reverting to the older kernel, you provided the system with a stable environment to completely repair the unsuccessful update.