Sewing machines, while designed to make sewing an easier task, sometimes malfunction in strange ways. If your sewing machine is only sewing in reverse, there are several factors that may be causing the problem.
The feed dogs are located beneath the needle and presser foot and move forward or backward to feed your fabric through the machine smoothly and evenly. Occasionally the feed dogs can get stuck in a reverse-feed direction and, no matter what setting you have your machine on, will only run backwards.
The reverse lever on a sewing machine is most often a push-button or mechanical lever that you push when you want your fabric to feed in reverse. This button occasionally gets stuck or broken internally in the reverse position, causing the machine to only run in reverse.
Modern sewing machines can have an on-board computer system that tells them what to do and how to do it. Like all computers, they can malfunction and wreak havoc on your sewing machine, including having it stitch in reverse only.
For any of these problems, it is best to take your sewing machine into an authorized service center to have it diagnosed and repaired.