Possible just restarting the application will cause it to reload properly:
$ killall gnome-software
Then launch the Ubuntu Software application.
If that doesn't resolve the issue, ensure that the issue isn't with the apt cache:
$ sudo apt clean
$ sudo apt update
$ sudo apt upgrade
Then launch the Ubuntu Software application.
If the above doesn't resolve it, then the steps will vary based on which version of Ubuntu.
With Ubuntu 16.04:
Remove the storage used by gnome-software:
$ killall gnome-software
$ rm -rf ~/.local/share/gnome-software
Then launch the Ubuntu Software application.
If that still doesn't resolve it, try to reinstall gnome-software
$ killall gnome-software
$ sudo apt gnome-software
$ sudo apt install gnome-software
Then launch the Ubuntu Software application.
The above suggestions are adapted from the article Ubuntu Software Center Not Working? Here Are the Fixes.
With Ubuntu 18.04 and higher:
Remove the storage used by the snap-store:
$ killall snap-store
$ rm -rf ~/snap/snap-store
Then launch the Ubuntu Software application.
It might not hurt to log off and log back in (or reboot even).
sudo apt-get update
Without further information I can only assume what's wrong. (Best guess is attempt to run command with software center still open (lock file)) – Elder Geek Aug 04 '16 at 15:53