Prevent OpenVPN from disconnecting (with PIA) via CLI?

I set up Private Internet Access via these instructions (OpenVPN) a few weeks ago. Everything seemed fine,

 wget http://ipinfo.io/ip -qO -

was reporting an IP from the expected location. Today, however, I discovered I wasn't connected anymore.

I want to prevent this from happening again. How can I ensure that my Ubuntu machine always stays connected to the VPN? If the VPN goes down, I'd rather lose internet access than connect directly through my ISP.

N.B. I'm running Ubuntu server (no GUI) and SSHing in. I don't need remote access (over the internet), but I do want to ensure I can still connect locally if the VPN disconnects.

If possible, please include instructions on how to persist changes after reboot.