It may be the chlorine content in your water has gone up.
Horses can tell even the most subtle changes in chlorine content and this can cause them to stop drinking the clean water you put for them.
I think you can get tester kits but i'm not sure where from.
Ask others on your yard if their horses have stopped drinking, if they have this could be the reason why.
My horse is very particular about what she drinks. She much prefers the disgusting water she gets out of the trough in the field and wasn't drinking at all in the stable. The way i get around this now is to only clean out her stable bucket every 3 days, i just top it up each day. It looks like she is unloved, but she drinks most of her bucket each night now. Oh, and believe me she is loved and it takes a great deal of will power not to clean the bucket!!!
Just an idea