Just a guess but I would think a small piece of gunk came free because of the oil change and stopped the passage to the gauge tube. You could back-pressure it with air to see if it frees the tube. One of those cans of air for a computer cleaning might be enough pressure. Just for your own information, wrong place for a gauge to be connected. If a crank or rod bearing fails will you be informed? If for some reason oil fails to reach the cylinder head will you know it before the followers inform you? Placing a gauge at the source only tells you the source is working, not what's happening along the rest of the path. In other words you will be informed that the pump is working but not know you have a problem until it is probibly too late.