Possible Abandoned Duck Nest?

I have a duck who for the second year in a row has laid eggs in my front flower bed. She laid her eggs around March 30. I know it takes about 28 days for them to hatch. She's been on the nest day and night as I always check on her. Yesterday morning I noticed a broken egg shell outside the nest and by her nest. I'm not sure if it was part of the same egg. I found her outside the nest pacing back and forth. After a few minutes she did go back to the nest. I checked on her a few times and she seemed to be OK. Also when I looked her eggs were uncovered. I'm sure that was done by the predator. I'm not sure if this happened during the day so I'm thinking it might have been someone's cat. I have no pets. When I went out early this morning to check on her, she wasn't on her nest. I've been out a few times to check on her and she's still not back. Do you think she's abandoned the nest? I'm so upset and very worried about her. Her male companion usually comes to visit in the morning and I sometimes see him on the front lawn, but I haven't seen either one of them today. What do you think?