Does your garage door keep coming back up after it hits the floor? You just need to adjust the travel setting on your door opener. The travel controls how far the door goes up and down. Most newer openers have a safety feature that makes the door reverse if it encounters an obstruction. Right now, your opener is reading the floor as an obstruction.
All openers have travel adjustments: Chamberlain and Craftsman openers (like the one pictured) have two set-screws on the back or side of the motor housing. Stanley openers usually have two knobs on the bottom of the motor housing. Genie openers have trip levers on the travel bar that you can reposition to adjust the travel. Adjust according to the instructions for your brand of opener.