Easily fixed with a few screws or nails.

I have also seen cases where two boards being butted together are not
over a joist but use the tounge and groove edges for support. Eventually
the tounges give way and the board flexes ... over time the tounge gets
ripped back to the last supporting joist.

This is not so easy to fix with screws and nails alone.

Also, when replacing screws and nails be careful where you screw. I tend
replace with a nail/screw of about the same length and use the same hole
if possible as wires and pipes are often notched into the top of joists
(they are *usually* in the centre ... but you never know).

If you can do so without damageing things further, lifting the board and
having a look is a good idea ... but bear in mind that lifting and
replacing boards will often cause more squeaks.

Alex