If you have an An Automatic Transmission,Your low on Tranny Fluid, or your tranny has finally passed on. Try put more Automatic Tranny Fluid in it. If that doesn't work Take it to a shop to have it rebuilt.If you have a Manual Transmission Your Reverse Idler gear might not be engaging are stripped(if you slam it hard, it finally killed that gear, That and possibly 2nd.) Make sure you fully press it in the reverse gear and listen for noises of engaugement, gear sliding in, etc. if you hear no sounds at all. or Nothing is working. Take the transmission to a repair shop. Or you can rebuild it yourself(DIY article on Civic-Eg.com, Under Transmission).