Hello! Do you mean two people aren't connected on the family tree as partners/siblings/parent-child anymore?
If this is the case, this should be quite easy to restore: In the case of a romantic relationship, just click on one of the partners in question, go to "Add partner/ex" on the sideboard and then "Add partner/ex already on tree". For a filial relationship, you need to have the siblings to have parents to connect them. If you don't, add parents to one of the siblings via the "Add parents" button on the sideboard, and then select "More Options" on the sideboard for the other sibling. Click on "Set parents" and choose the parents (or only one, if they are half-siblings) the first sibling has.
Or is this a display issue? Your family members are still all displayed in their current positions but it is literally just the lines lacking? If that's the case, maybe your line settings were somehow messed up and you could go to "Options" in the menu on the bottom of your screen and edit/reset line style.
I hope this helps - if I didn't understand your problem correctly, could you give more information? Otherwise, you can always use the "Send Feedback" option to give Feedback about bugs to the Support of FamilyEcho.