# External monitor not detected on Ubuntu 20.04

###### tags: `Linux`

1. update kernel 5.10->5.14
2. reboot -> found that one monitor works only
3. check if gpu conneted ```lspci | grep 'VGA'```
4. check gpu driver ```nvidia-smi```
5. message: ```nvidia-smi has failed because it couldn't communicate with the nvidia driver. make sure that the latest nvidia driver is installed and running```
6. check kernel version ```uname -a``` -> ```5.14```
7. force to use nvidia gpu ```prime-select nvidia```
8. noticed that [someone](https://forums.developer.nvidia.com/t/ubuntu-doesnt-detect-my-second-hdmi-display/75076/6) said that the driver is built for specific kernel version
9. start to downgrade the kernel version :cry: using gui tool

sudo add-apt-repository ppa:danielrichter2007/grub-customizer
sudo apt-get update
sudo apt-get install grub-customizer

10. change the grub boot order to the older one, save, and reboot
11. reinstall the driver

sudo apt install nvidia-cuda-toolkit
sudo apt-get purge 'nvidia*'
sudo add-apt-repository ppa:graphics-drivers
sudo apt-get update
sudo nvidia-smi

12. log out and log in
13. fixed!