What do I do now that the grass has taken over my day Lilly plant?

pull the grass out or smother it out with newspapers, cardboard,heavy landscape fabric and top with mulch

Dig your plant up, pull all the grass from around it and make a clean space and re-plant it. If it’s really large you can separate it and plant somewhere else. Lillie’s are pretty hardy.

Dig the whole plant up in order to pull out the grass, then make a clean spot, and replant. If it’s really large you can divide it up and put some in different spots. Lillie’s are pretty hardy.

You can possibly dig it up and clean the grass from the "roots". Then you can transplant it where there is no grass.