Well... in my experience, sour has meant a bacterial infection, most likely lactobacillus. My problem was caused by reusing yeast too many times (and probably waiting too long in-between reuses). Cleaning or sanitization problems are the usual cause.
If it is lacto, there is nothing to do about it. It gets stronger until it is undrinkable, to my tastes. The beer, however, was fine to use for cooking (boiling bratwursts, etc.).
If it is still drinkable, but just a little sour, refridgerate it all as cold as possible immediately - that seems to slow it way down and maybe you can finish drinking the batch.
(If it smells/tastes like vinegar, then it may be acetobacter instead - but that usually needs a lot of oxygen to develop, ie. a container open to the air.)
Derric