Code U0100

There was a small shudder in the idle to begin with, just on a short trip to the grocery store. A few days later after driving about 30 minutes out and spending a few hours hiking and then starting the car again (yesterday) the check engine light suddenly came on and I was able to drive the car to a friend's house with no issue. We checked the code for the light and it came up with just a U0100, having issues connecting to ECM or PCM. The car wouldn't start or crank afterwards, also would not jump so battery doesn't seem to be the issue. After looking at it a while (20 minutes or so), we tried starting it again. The car started but all of the gauges were either not working or not reading correctly as well as service errors coming up like power steering and traction control that aren't really a problem, especially since U0100 is the only code coming up and will not clear. After another hour the car started again, this time everything seemed normal. We did not drive it anywhere for fear of it shutting off while driving. What could be the problem? What sort of things should we check?