Every wooden fence has a lifespan and that can, unfortunately, be shortened by a number of factors. If the fence has been damaged by the elements, or an errant football, or has been in constant and direct contact with the soil, things may rot earlier than expected. Repair is possible.
Fence repair
If a whole section of the panel is rotten, try easing it away from the supporting posts using a crowbar. Gently open up a gap between the post and panel to expose any nails. Then you can simply hacksaw through the nails to allow you, and a friend, to lift the damaged panel away. It can then be repaired by sliding out individual slats and replacing with new. Remember to hammer down the nails left in the post or pull them out using pliers before refitting the repaired panel.
Broken fence posts
Wobbly fence panels are often a sign that a fence post has given up the ghost – but it too can be repaired. Spurs (not Tottenham) can be fitted alongside the post, sinking into the soil with the sound part of the post affixed to this solid support spur. Complete rails can also be easily repaired. Metal spurs can again be used to connect sound wood to sound wood negating the need to either replace the whole rail or even the complete panel.
Replace the fence panel
Obviously, if any damage is beyond repair, or the repair itself is prohibitively costly, it may be better to replace the whole panel or post completely. Just make sure that you erect the panels and posts correctly to get the best from them over the longest time possible. And perhaps ban football in the garden (never!).