ink and printer

why does the computer say that the printer is out of ink - yet when I removed the ink cartridge, the container is still 90% full of ink? Earlier, it said the printer was out of ink, and those cartridges were still 30% full of ink.

Are these cartridges really out of ink?

Has the printer been sitting without use for a couple of weeks? If so, the printhead on the cartridge is probably clogged and now useless... Which is why it feels full yet not functioning.