If it's happened to you, you are not alone, this is a common problem with older opener. There are a few reasons why a Broken opener gear or worn out Drive gear may occur.
1. The chain was installed too tight, and over time (usually few years), it applied too much pressure on the gear, causing the metal to grind until it snaps.
If you see the chain loose on the rail, you probably have a broken gear.
2. The door is too heavy, which usually means there is a problem with the spring, either a lack of tension or a broken spring.
This problem is easy to identify. Try to open the door manually. If it goes down by itself and doesn't stay balanced, you have a problem with the spring.
3. The gear was subjected to heavy impact. For example, you try to close the door while the openers force limit is on the highest level, but
there is something in the way.
4. Its just too old! When an opener is 15+ years old, it isn't surprising to see problems with it, one of them being a broken gear.
With time, parts such as metal and plastic get worn and eventually break.