Although the gear and sprocket has been replaced, the whole reason this part has failed is because the springs have lost their initial tension and the door is no longer balanced. This should be addressed with your garage door service technician.
At this point, it would be wise to call a garage door company to change the gear & sprocket out. There are a few different types of gear & sprockets depending on the garage door opener model that you have. It is a pretty simple fix for a qualified technician and usually costs around $120-$150, which is a whole lot cheaper than having to replace the whole garage door opener.
When the bushings collapse on the top plate, the chain or belt becomes loose causing more friction on the gear & sprocket. Eventually, this will cause the sprocket to snap off. This gives the appearance of a broken chain or belt.
One of the most common repairs on a Liftmaster garage door opener is a broken gear & sprocket. This is the part that connects the motor to the chain or belt. After several years, the garage door springs become weaker causing more stress on this particular part causing it to fail and wear out.
Now on to the other settings, the ones that make the door stop and reverse if the door comes down on something. These are a second safety precaution that tie in with your safety sensors at the bottom of your garage door. If something drops into the way of your garage door, that is above the line of sight of your safety sensors, then these settings are what causes the door to reverse.
Once the garage door technician has changed the gear & sprocket, he will then need to set the up and down limits on your garage door opener. This is what tells the unit how far to close the garage door and how far to open the garage door. These are usually on the side of the garage door opener as shown in the picture.
As the bushing is getting worn out on the top of the unit, inside of the garage door opener, the teeth on the nylon gear are grinding down. As you can see in the picture, once the teeth are worn down on the gear, it will no longer turn the drive gear that moves the chain or belt. You will be able to hear the motor running but the garage door opener, or in our case, the gear & sprocket will not move the garage door up or down.