This problem comes up periodically with nordvpn updates. It has happened to me four or five times in the last year alone. I'm currently running Ubuntu 18.04.3 LTS but I have experienced these same issues on 18.04.2 LTS.
Here's how I get up and running again.
Note, this solution assumes you running a version of Ubuntu 18.04 and that you have a usable internet connection.
Step 1. Open your terminal.
Step 2. Run
Step 3. Run
sudo apt purge nordvpn.
Step 4. Run
sudo ls -la /var/lib/ and confirm that the nordvpn directory has been removed from /var/lib/. If it's still there we might run into problems later so go ahead and run
sudo rm -rf ./nordvpn/ to get rid of it.
Step 5. Run
sudo apt install nordvpn. If apt has any issues installing, open up your web browser and navigate to https://nordvpn.com/download/linux/. Go ahead and download the .deb file and install it. Instructions for that process should be available on the download page.
Step 6. Run
nordvpn login and enter your NordVPN credentials.
Step 7. Run
nordvpn c. After a couple of seconds you should see something like Connecting to United States #XXXX (usXXXX.nordvpn.com). If it succeeds you should see something like You are connected to United States #XXXX (usXXXX.nordvpn.com).
And with that you should be good to go.
If you are having this problem on a different version of Ubuntu then I recommend looking for a different solution before trying this one. Good luck!