Veg plot has become sterile?

I have a little veg plot in my garden, about 5 X 4 meters. 7-8 years ago it was producing plenty of excellent results. Basic stuff…lettuce, carrots, peas, spring onion…but it was always great fun. Last year several of my crops started off well, but then completely failed, and this year pretty much nothing grew, except the carrots…and they’re not looking good. Any ideas of what’s happened and what can I do to remedy?