Lobster wrote:

Might be
that the board was cut across it's width (by the electrician?!) and has
nothing under it to support it; in which case hopefully it's right next
to a joist, then you can lift the board and screw a noggin (ie a block
of wood) to said joist to act as a support for the sagging board.

...and if it's not next to a joist you can screw a noggin/support under
the two adjacent boards.