There's no simple answer. If the remote has been sitting unused for a while, the battery may have discharged to the point of rendering it useless. All rechargeable batteries have a lifetime that depends on multiple factors. You can probably find a replacement battery on eBay if you're able to open the remote. It's a real pain, but it just snaps together.
You can try the old reboot the remote routine by holding the pairing button for 20 seconds or pressing Home and Back simultaneously for 20 seconds, but if the battery is toast, it won't do any good.
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