Erik By hidden - do you mean marked complete?
If you marked them complete, then they “disappeared” it was because the filter is set to show only items that are not marked complete (this is the default filter setting).
The way to view them is to adjust the filter: Filter box > status > complete (the check mark icon).
Clicking that check mark icon will cycle through:
Red = show only not completed items
Blue = show only completed items
Grey = show all items regardless of complete status
