Well, I'm probably sounding repetitive, but it needs a new battery. If you end up not changing the battery for a while, the screen will go blank. This doesn't mean your tama is broke...that's what my brother thought. The battery's a 2032, I think.
Yes, before you throw away your battery you can try the reset button on the back. Then press the B button and choose DOWNLOAD. Sometimes this will keep the tama going for a few more weeks before the battery is really and truly dead.
I have a faulty tama where even a brand new battery shows the X; then I reset all day for a day or so. Then it is fine.