UPDATE: Reading through one of the bug reports I was able to get mine back up and working with the following steps. You might want to try it as well. This is only a temporary work around, as it doesn't last too long and you would probably have to run it again.
Open a terminal window Ctrl+Alt+T and run the following lines:
pkill gnome-software
sudo rm -r /var/cache/app-info
sudo appstreamcli refresh --force --verbose
Then relaunch the gnome-software
center.
Unfortunately this is a bug in the gnome-software application. Here are a couple so far that I was able to find related to this:
https://bugs.launchpad.net/ubuntu/+source/gnome-software/+bug/1589970
and
https://bugs.launchpad.net/ubuntu/+source/gnome-software/+bug/1579415
As an alternative you could use the Synaptic Package Manager.
sudo apt install synaptic
Or you could use the original Ubuntu Software Center that still works just not fully updated (No longer available in 18.04):
sudo apt install software-center