Rechargeable batteries, like the one in your Roomba, can degrade over time, especially if left unused for extended periods. This degradation can lead to a reduced ability to hold a charge. If the battery is completely discharged, it might have reached a state known as "deep discharge," making it unresponsive to charging.
According to what I found online, the only option when a Roomba's battery is deep discharged is to replace it. However, before you jump to a replacement you can try resetting the battery:
# Turn the Roomba over on its back
# Remove the battery
# Turn the Roomba back over
# Press and hold the power button for five seconds
# Turn the Roomba over on its back again
# Reinstall the battery