One day you notice that your sound doesn't have the same sparkle it used to, but you aren't doing anything different when you play. You feel like you're getting worse without even trying!
Rest assured that your sound hasn't changed at all; your ear has, and this is a good thing.
Ira Glass has a great take on this phenomenon. It's only a couple minutes long, check it out:
Assuming that you practice regularly and thoughtfully, you're going to hit the wall every now and then. For what seems like no reason at all, some part of the way you play will seem terrible when it used to be fine. This is because, like Glass talks about, your taste has become better than your ability. Your ear has refined itself and you have higher goals now.
So you work hard to get better. But by the time you improve enough to meet this new standard, your ear has also had time to find new things to hear that it didn't notice before. You hit the wall again and start over.
But here's what you have to remember: it only feels this way to you. Only you notice when you hit the wall. To outside observers, you just keep on getting better and better.