Crazy readings

[sj1000]

This is strange, We've had our pool for 8 years now and have had no problem keeping the chems right. But now I can't get chlorine to register! After shocking several times, when I test the water what should be the normal reading for chlorine, it turns green, not even a hint at the nice pink it should be. There is algae in the pool but usually it would be killed with a couple of shocks. I'm stumped