The longer answer
Why it went all at once
Torsion springs are rated in cycles, not years. A spring rated for ten thousand cycles on a door used four times a day reaches the end of its life in about seven years, and when it goes it goes suddenly and loudly. Most people describe it as a bang from the garage.
Why the door is now too heavy
A double door weighs somewhere between 150 and 250 pounds. The spring carries almost all of that, which is why a balanced door can be lifted with one hand. With the spring gone there is nothing between you and the full weight.
Why we replace springs in pairs
If one has reached the end of its cycle life the other is within months of it, and a new spring paired against a tired one wears out faster than either would alone. Replacing both is cheaper than two visits.
What a visit involves
A technician measures the door and sizes the new springs to its actual weight, replaces both springs, retensions them, and tests the balance and the safety reversal before leaving.