I had this happen multiple times, at some point, simply all connections on my schematic to a power net or to ground disappear. I believe it happens when a change has not yet fully completed and then I hit "undo". Typically this happens when trying to adjust a connection, and after making a change, the editor will select the entire wire even though I intended to just select a single wire element and tried to move it. This ends up moving the entire wire along and sometimes creating loads of meaningless nodes. Unfortunately, trying to reverse to a previous version results in an error message prompting me to file a bug report.