You could try and incubate it if you think there is a chance it is fertile, but if she has not been kept with male turtles then it is likely to just be an infertile egg. It's not something to worry about, but provide an area out of water with sand or other substrate for her to lay eggs in (I assume she laid this one in the water?). If she hasn't been provided with a nesting site before, she may well have laid eggs in the water in the past and just eaten them before you spotted them
Offer her cuttlefish bone to boost her calcium, she will have used a lot of her calcium resources for making the egg, and will need more.
Best,
Paul