Wooden floor has sunk in dining room

May be it look like dry rotten It's better to rectify ASAP otherwise it's spread So best way hire someone to check

Hi definitely sounds like an issue with your joist the simplest thing wrong is that packing has come loose. If your air vents under the floor have been blocked then it's possible you may have rotten joists. If you catch early enough you don't necessarily have to replace the whole joist but cut the rotten end off and fix a new piece on or fix a trim and trimmed joist with joist hangers. Suggest removing several floor boards around the affected area and having a look. Hard to tell without looking but hope it helps.