There's a short range NFC chip in there that should last indefinitely. That's the one used for getting into the parks, tapping for room charges, getting into your Disney hotel room, photopass, etc.
There's also a long range RFID chip that does things like show your name on small world. That one lasts about 2 years, but it's also dependent on how long your Magicband was on the shelf before getting it. So people say it's reliable for about 1 year.
If this is one of the original bands, that battery is now dead, and the RFID part won't work anymore. If it's a new band, you can buy a $15 plain magicband of the same base color, and using an eyeglass screwdriver, and swap the pucks.
Make sure to register the new puck to your MDE account before using it.