Suddenly none of my buildings are connected to a path - what happened?

If all your buildings were connected to a path before but all of sudden you see lots of bubbles with red circle with lines through them, it's possible that a path piece was accidentally deleted. Go into edit paths mode and have a look at each path piece. See if there are any gaps you need to fill by adding an extra path piece. It's worth remembering that in order to be connected to the main path network, it needs to be possible for a guest to walk along a path from reception to the building's entrance.