Is placing the hands before the knees when prostrating correct?

If what is meant is that you used to place your knees before your hands during prostration (sujud), and then you started placing your hands before your knees after your fall, then your prayer is valid and there is no harm upon you. The difference of opinion among scholars regarding whether to place hands or knees first during prostration pertains to what is superior (afḍal) and the Sunnah, and the matter is broad, especially in your situation.