No power the player is charged fully but won't play music. could this be the battery? it is about 2 1/2 years old.

The internal battery has failed in storage. As you can imagine the internal Lithium battery is tiny. You can try a reset, but it needs to go back under warranty if that doesn't fix it. The reviews are mixed..

https://www.google.com/search?q=Finis+Duo+underwater+MP3+player+won%27t+hold+charge

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how do you know it is fully charged? i think it may be dead battery

MP3 players can only play music files in mp3 file format.You can download mp3 music files from the Internet toyour computer and then copy them to your MP3 player.For your CD collection, you need to convert your CDs tomp3 music files onto your computer or download them from the Internet and thencopy them to your MP3 player after you have connected the USB cable tothe MP3 player and the computer.
You can do this using Windows Media player and Rip the CDs - Media Player willcreate a MP3 music file for each music track.Click on this link for instruction on converting musicfiles to mp3 file format :- http://www.fixya.com/support/r5798418-creating_transferring_mp3_music_files

If the MP3 player battery is more than 3 - 4 years old then the battery could be worn out.Rechargeable batteries have a finite number of charge and discharge cycles andwill lose their charge capacity over time, i.e. won't charge to 100% andgradually the charge reduces until the battery won't charge up at all. OR Thebattery shows a 100% charge but when the adapter is disconnected the batterydrops off to zero capacity in a very short time. If the battery drops to anunacceptable charge level then the battery needs to be replaced.

Ifthe rechargeable battery is more than 3 - 4 years old then the battery could be wornout. Rechargeable batteries have a finite number of charge and discharge cyclesand will lose their charge capacity over time, i.e. won't charge to 100% andgradually the charge reduces until the battery won't charge up atall. ORThe battery shows a 100% charge but when the adapter is disconnected thebattery drops off to zero capacity in a very short time. Ifthe battery drops to an unacceptable charge level then the battery needs to bereplaced.

The Batteries in these units like all rechargeable batteries only have a certain number of times they can be charged and discharged before begining to fail. Your only option is to replace the battery with a new one. They are cheap to purchase off eBay and YouTube has Videos detailing how to change them.

No it doesn't have to be fully charged as long as there is enough battery to turn on and play music it'll turn on it'll also remind you if the battery is low

Reset the player then try playing songs. Ensure that the battery is fully charged. You may also try formatting the player to reload musics. If the issue persists then get the player serviced.

Did you remove protective packaging on batteries?

I've called Apple support on this before. They told me to make sure that the Ipod is set for Manually manage music in the Ipod settings of ITunes. You should restore all software in your IPod to factory settings before doing this.