Sometimes wooden slats supporting the box spring for your bed do break. Most often, the break occurs when children are jumping on the bed repeatedly or during a move. Under normal use, the slats should not need repair. A broken slat is easy and inexpensive to fix. It is important to fix the problem because your mattress and box spring are not supported properly in that spot and could be damaged.
Measure the length and thickness of a good slat.
Buy a wood board as close as possible to that length and thickness. Pay particular attention to the thickness.
Cut the board to the proper length.
Place the new slat on the bed frame. It should fit approximately the same as the rest of the slats and your box spring should rest comfortably on all slats.
When you buy the board, ask if the store will cut it to the proper length for you. Most home improvement stores will cut a board at no extra charge.