What year is your Volt? I had a similar issue with my 2012. If I remember it probably happened around 2013.
I got message reduced propulsion -engine not available. I pulled off the highway and parked. Tried restarting-no engine just funny noises. Onstar sent a tow. When the tow got there, I was able to start with the engine running. I drove 50 miles to my closest dealer and left it, after hours. They had it for 3 or 4 days. They never found anything wrong with it. It has been fine with these regard since. The story that I got was " it probably a metal shaving from production that got caught in a transmission solenoid, which then probably dislodged hopefully getting caught in a filter"
Maybe you should try restarting after it sits a little while. If that does not work, you should probably at least get codes.