Established pool having sudden trouble with green cloudy wat

[Saddlefiend]

Hi,
We have a 32 foot x 14 ft x 5 ft oval pool, it is established and we've never had a problem with the water. It's usually crystal clear and lovely. The last week it has suddenly gone green and we've had a heckofatime keeping chlorine levels up in it! No changes is chemicals or in chemical brands or anything like that, same water filling it as in years before. PH is also low....we are at a total loss. Hubby is throwing chlorine at it like mad and also vaccumming and backwashing. HELP....
Chlorine level is unreadeable, and PH is just below desired levels (finally!)