Rugs, Carpets and Carpeting - Carpet over old stairs is sinking

The carpet in our dining room was laid over stairs in one area. That section (2x4, roughly) is now sinking and very obvious. When we bought the house three years ago, I asked the inspector about it and he dismissed it as a problem. But it is. And the sinking has only intensified with all the traffic going across it.

Is there anything quick and inexpensive that can be done to fix this? I have no idea what floor is underneath the carpet in the area of the stairs.

I'm unclear here. Are you saying you have a carpeted set of stairs that are misbehaving or an abandoned set of stairs that had a floor built over them and carpet laid over that?

It's a set of abandoned stairs that carpet was laid over, so about a 2x5 section of the carpet is indented and noticeably sinking from where it's been walked on over time. We're about to pull it up to see what the subfloor is underneath and we're wanting to know what we can do to correct the problem so it doesn't happen again.

Carpet sinking or floor sinking?

If the abandoned stairs go down to a basement or area where you can inspect the board and provide additional support, that is a possible option. If not, then carpet will have to be removed. If plywood was placed over the opening in floor where there once was a set of stairs, then board can be removed and wood sistered to joists below to provide additional support.

If the board is sinking, then this is a safety hazard. When the board eventually gives way, someone will likely tumble down the abandoned stairs.

It sounds to me that you'll find, when they covered the abandoned stairs, they weren't careful to make sure the new section of floor was level with the old and the new section is a bit lower due to using thinner material than the existing floor. This would cause a trampoline effect and the carpet over the lower area would stretch out more than the surrounding carpet due to more stress on it than the rest. Over time, the edge of the repair would begin to show through the carpet with a distinct wear pattern and the carpet over the old stairs would start developing wrinkles in it. The fix is to do whatever is necessary to make the entire floor the same thickness under the carpet. You'll probably need to pull up whatever they put in and replace it with the proper thickness material. Once that is done, there won't be a trampoline effect and the carpet can be re-installed. I believe I would have it professionally cleaned afterward. That will restore the carpet appearance as well as can be expected.