If your track is heavily worn you will be able to install it back onto the machine without opening the track. We usually put the track on the sprocket and roll it back over the front idler. You can usually use a chain and the bucket to help with this. If your chain is still in decent condition, you will need to open the track by removing the master pin then install it on the machine and hit your master pin back in. When a track comes off of a machine usually the cause is a blown track adjuster seal, wore out pins and bushings, or the tension wasn't adjusted correctly. Be sure to find out the reason why your track came off and correct the problem to prevent this from happening again.