It sounds like one or more of the ordinate dimensions have lost their links to the model. Click on one of them to highlight it and see if there's a red box at the end of its leader. If there is then click on it and drag it back to the hole. If it reattaches then the problem is solved. Repeat as needed. If that doesn't work then you don't need to delete the whole ordinate set. Just delete the one (or ones) that lost its reference, right-click on the zero or any of the other dimensions of the set, and choose "Add to Ordinate" from the drop-down. Add new dimensions just like you do when starting a set from scratch.