DIAGNOSIS That's the battery's fault indicator. It means the battery pack's internal protection circuit has tripped, usually from a voltage imbalance between the cells or a thermal event.
FIX First, take the battery off the tool. Let it sit off the charger and the drill for a good 30 minutes to cool down and reset. Then, pop it onto a Milwaukee M18 charger. If it's a genuine fault, the charger lights will usually just blink or show an error and won't charge. If it starts charging normally after the cool-down, you likely just overheated it. Let it charge fully, then try it on the drill again.
IF THAT DIDN'T WORK If the charger still won't recognize it or it immediately flashes again on the tool, the battery pack itself is probably faulty. For the 2904-20, this is almost always a battery issue, not a drill issue. Try a different, known-good M18 battery on your drill to confirm the drill works. If it does, your original battery needs service or replacement.
HEADS UP Don't keep trying to force a faulty battery to work. A tripped protection circuit is there for safety. Continuing to try can prevent it from ever resetting and might be a sign of a failing cell, which you don't want to risk.