My taskbar on the left side displays the system monitor symbol. By just clicking on it, I was able to open it. But suddenly, it stopped working. I clicked on it and Ubuntu seemed to load and open the process but quitted doing so after a while – and acted as if I have never clicked on that button. Yesterday, everything was still fine. Today, Ubuntu is acting that weird.
First, I have tried to install the system monitor all over again by typing
sudo apt install gnome-system-monitor into my terminal. This did not work. I tried to uninstall it with
sudo apt purge gnome-system-monitor and install it again with
sudo apt-get install gnome-system-monitor. It still did not work. So, I just typed in
gnome-system-monitor and it worked. But it was displayed as a separate instance on my taskbar. So, I just deleted the old link and made myself a new one. Now, everything is fine again.