Why did my garage door cable break?

Garage door cables do a lot of work. They lift the door and counter balance the springs. In an extension spring system, they run on pulleys and in a torsion spring system they roll into a drum at the top of the door. There are many reasons garage door cables break but a here a few common ones.
1. Cables are rubbing against the track or other garage door hardware.
2. Moisture caused the cable to rust. Usually, moisture occurs at the bottom where water from rain or snow seeps in the garage. However, if you leave the door open during these weather events, the cable can rust anywhere.
3. The pulley could be splitting somewhere and causing the cable to fray. Invest in cast iron pulleys which are resistant to splitting.

CAUTION: Garage doors are a very big moving object and their parts balance each other out so that minimal work is required while moving. When one of the garage door parts break, it changes the system. If you try to lift the garage door while the parts are broken, you could be causing injury to yourself or damaging the other garage doors parts.