Is my frog lonely?

Two days ago I emptied my pond, two frogs jumped out. (never seen frogs there before) I quickly filled it up again, since then I have seen only one frog. It sits on some weed in the evening and croaks all night. What is all that about? Has his friend moved because of the disturbance? There is/was no spawn. Sensible answers please as I'm worried about the little chap.