OK take a deep breath, I've been there. Only it was 3/4ths of a pie and a CAN of readywhip
So you ate 1/2 a pie. Now you dont say what kind of pie so I am just gonna pick a random pie. A 2 crust 9" cherry pie. 1/2 of a 2 crust 9" cherry pie is roughly 1500 calories. Yes that is alot. But it is less than 1/2 a lb.
If you are currently losing weight at ~ 0.5 lbs/week then you delayed your goal by a week? Maybe less?
In the grand scheme of things its a bump and nothing more. Live, learn and move on.