It has been some months since I've turned on my Playstation 3. Today, when I turned it on with my controllers, it didn't work. I thought "Hey, the battery surely ran out", so I tried to charge them but they don't seem to be charging at all. Could it be that they're officially weight papers by now?
If your PS3 controller has been unplugged and out of battery for a while, it wouldn't be able to maintain it's 'sync' with your PS3.
You will need to attach a USB cable to the PS3 and the other end to the controller for them to sync up properly again.
The other side of this, it's that the batteries in the PS3 controller have potentially degraded to the point where they will no longer hold a charge. If you leave them charging, but they switch off immediately once being unplugged, this is likely the case.
You should still be able to play games with them so long as they are plugged in to the PS3, but long term it might be worth investing in a new controller.