YCM ceased working after Ubuntu upgrade.To make it work again, you need to use the vim-youcompleteme that comes from the apt repository and not the one from other plugin managers like VimPlug.

Here the error message that I got:

ycm ImportError: Python version mismatch: module was compiled for Python 3.6, but the interpreter version is incompatible: 3.8.2 (default, Feb 26 2020, 02:56:10) [GCC 7.4.0]

I solved the problem following the next steps:

1. Given that I use VimPlug, delete youcompleteme from my .vimrc.
2. Open Vim and run :PlugClean
3. sudo apt update
4. sudo apt-get install vim-youcompleteme
5. vim-addon-manager install youcompleteme