If you can’t fix it with in game action and you don’t mind cheats this cheat is available.
ModifyRelationship (Sim1) (Sim2) 100 LTR_Romance_Main
or
ModifyRelationship (Sim1) (Sim2) 100 LTR_Friendship_Main
Replace (Sim1) (Sim2) with sim names without parentheses. 100 gives full bars. Change the number to anything from 0-100 to give whatever positive relationship level you want. Negative numbers give negative relationships. I’ve had to relocate my save a couple of times and cheat lost relationships back in, so I know it works.