Why is my cake so dense?

[hellebore]

I attached a photo of my cake results. It's extremely dense but moist. It didn't seem under baked, did I put too much flour or over-mixed the batter? What are common causes for cakes resulting like this?

*Extra notes: I kept this in the fridge after coating it with Italian Buttercream overnight

 

[retired baker]

impossible to discuss without knowing what it is and how you made it.
Maybe its a boxed cake mix?
I wouldn't have bothered finishing it, I would have bailed and done it again.
It has the look of no baking powder or aeration of any sort.