How long is it inside?
Squat and bear down as one pushes down for the motion (as one is constipated). Please keep a mirror down, so you can visualize the condom. If you see the condom, remove it by fingers.
If it is not seen, you will have to consult a physician or nurse practioner to get it removed.
Please read this resource, which is for the lost tampon but would work for lost condom also.
Please feel free for your follow up questions.