Well, my friend, the provided context is about as helpful for fixing a sofa as a parachute in a submarine. It appears to be the generic footer from YouTube's website, which won't do much to stop that sinking feeling when you sit down. So, let's ignore that and dive into the actual guts of your couch.
First, you must become a sofa spelunker. Flip that bad boy over and face its underbelly. You'll likely see a thin fabric dust cover stapled to the frame. You can either remove the staples with the patience of a saint or rip it off like you're opening a gift on Christmas morning. The choice is yours, but only one is considered "proper."
Once you've gained access, you can diagnose the issue. Most modern sofas use S-shaped, zigzag springs. Often, the problem isn't the spring itself but the little clip that holds it to the wooden frame. If a clip has broken or come loose, you can simply pry the old one off and install a new one with a hammer or staple gun. If the spring has just popped out of its clip, you'll need to use some serious muscle and a pair of pliers to stretch it back into place. Be warned, that spring will fight you like a cornered raccoon.
If a spring has actually snapped, you'll need to replace it. Measure the old spring before you order a new one. Unhook the broken pieces from the clips at both ends of the frame, then wrestle the new spring into position. It requires a bit of leverage and a lot of grit, but the feeling of victory when your couch no longer has a designated sinkhole is well worth it. Once your repair is complete, just staple the dust cover back on, and no one will ever know about the battle you waged beneath the cushions.
