Well, let's start by addressing the provided "context." One source is the generic footer of a YouTube page, which is about as useful for furniture repair as a screen door on a submarine. The other is a link to a Reddit post about a quick fix. So, with that wealth of non-information, let's toss it aside and get down to brass tacks based on actual know-how.
So, your sofa has decided to impersonate a hammock, has it? That sinking feeling you get isn't just existential dread; it's a broken spring. Before you surrender your beloved couch to the landfill, let's perform a little couch surgery.
First, you'll need to play detective. Manhandle that sofa onto its back or side to expose its underbelly. You'll likely see a fabric dust cover stapled to the frame. Gently pry those staples off to reveal the sofa's guts. You'll probably see one of two main culprits: S-shaped metal pieces called zig-zag or sinuous springs, or a grid of traditional coil springs.
If a zig-zag spring has simply popped out of its clip, you're in luck. Using some muscle and a pair of pliers, you can often wrestle it back into place. If the clip itself is broken, you can buy replacements online or at a hardware store. If the spring has snapped entirely, you'll need to replace it. Unhook the broken pieces, measure the length of an intact neighboring spring (don't measure the broken one, you clever clogs), and order a replacement. Getting the new one in requires some serious pulling, so a spring-pulling tool can be a real lifesaver, but brute force and pliers can also work if you're feeling feisty.
For coil springs, the job is a bit more involved as they are often tied together with a specific pattern of twine. If one of those ties has broken, you might be able to re-tie it with new twine. If the spring itself is shot, replacement is more of a project, and you might consider calling in a pro unless you enjoy a good challenge and a potential new hobby in upholstery.
Finally, if all that sounds like too much effort, there's always the time-honored "student" fix: cut a piece of plywood to fit under the cushions. It's not elegant, but it'll stop the sag and save your backside until you're ready for a real battle.
