What's wrong with my nikon 5100 battery?

For the past day or so I've plugged my battery into the wall charger and the light would show up blinking red. I don't remember if when it charged correctly if it flashed green or red. Last night I left it in to charge all night and I put it back into my camera this morning and it said that the battery died. In the manual it says nothing about a dead battery so I'm just wondering if anyone has any idea what is wrong and where I can get my hands on a new battery.

I have the D5000. I think the light stops flashing when the battery is fully charged. It's possible your battery is just worn out.
You can look on line at places like amazon or ebay to find a new one. Figure on around $35 for a new one. Be sure to only buy a Nikon brand battery & avoid the cheapy batteries. They're cheap for a reason.