One of the most common is a water damaged floor. This can be the rult of many mishaps, ranging from a leaking pipe to a spilled drink to a leaking roof to the absolute worst, a flash flood. A water damage floor needs to be inspected, and covering materials unraveled. Because wood rot and other damage may actually be beneath a carpet or other covering, you may not notice the damage until it is very bad.
Rotten wood will be more spongy than hard wood, and should not echo well if knocked. Sometimes there is no evidence of water damage, and yet you might expect it because something was spilled there, or repeatedly.
professional help is a very intelligent option for this type of project. A professional knows the many complex steps and safety proceedures that are necessary.