I recently installed Ubuntu 20.04 LTS and update and upgrade it and just installed vlc.
after I reboot my PC and log in again everything is fine but it hangs if I go and click to open file manager or terminal or anything it will stuck and nothing works and some parts of my monitor start blinking then I cant just reset the system. Is this problem from the last update? how I can dis upgrade and back to the last version? please help.
There have been several questions about this same problem posted at Ask Ubuntu today. From the recent update history it could be caused by either a kernel update or an Nvidia proprietary graphics driver update. Your computer doesn't have an Nvidia graphics processor, so try booting with an earlier kernel version. Select Advanced options for Ubuntu in the grub boot menu and boot with an earlier kernel version.
If this solves the problem then make it so that you don't have to select that older kernel version every time you want to boot Ubuntu. Make a note of which kernel version was causing the problem. Uninstall the kernel version that was causing the problem. Then prevent the older kernel version from upgrading to it again by following the instructions in the answers to Blacklist only one specific version of a package for upgrades.
Alternatively there is already a new kernel in both focal-proposed and groovy-proposed.
If your hardware has a problem with your current kernel or you just want to use a lower kernel version remove the latest Linux kernel version (for example linux-image-5.4.0-91-generic).
sudo apt purge linux-image-5.4.0-91-generic linux-headers-5.4.0-91-generic linux-headers-5.4.0-91
For a GUI way of doing the same thing you can install Grub Customizer with sudo apt install grub-customizer in Ubuntu 20.04 and later. In Ubuntu 16.04 and 18.04 you can install Grub Customizer with the following commands:
sudo add-apt-repository ppa:danielrichter2007/grub-customizer
sudo apt update
sudo apt install grub-customizer
In the General settings tab of Grub Customizer click the dropdown menu for predefined, which lists all the available kernels. Choose an older kernel and click Save.