Mp3 player will not turn on, it is fully charged

get a new one.

That would depend on the make and model of the player. I know my MP3 player is fully charged when the battery gauge shows all green.

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

How to charge the Jensen MP3 player - Model smpg-v2bub

Ignore instructions that came with it - they don't work.

Here's how to charge - I discovered this with mine:
1. When computer is on and at windows screen, attach cable from computer to mp3player with mp3 turned player off. It will immediately show you a connect screen on the mp3 player.

2. pressed the left side middle button twice - this will turn it on and go into music like you're playing a song.

3. Now turn the mp3 player on by switching the button on the right side of the device to the on position. It will suddenly show you it's charging - the battery on the front of the mp3 player will have a signal pulsing through it.

4. It will turn off after one minute. It's literally half charged now. Strange I know. Repeat from step 1 and it will fully charged.

If it doesn't fully charge after it turns itself off after 1 minute the 2nd time, then repeat steps from 1, one more time, but this time when it turns itself off after a minute, still leave it connected to the computer for 10 mins even though it looks like it's off. Then disconnect it physically from computer, turn it on with the on button on the right side, press the left side middle button twice and look at the battery on the screen - it should be fully charged now.

If you can turn your player on while being plugged to your PC/Laptop or USB charger, then your internal Lion battery is most likely gone past its useful life (some 500 charges or 18 months usage) and may need replacing. Most MP3 player may show 'fully charged' when plugged in either when the battery is truly fully charge or defective battery is no longer taking a charge: e.g. minimum or no current flow situation.

once your mp3 player gets fully charged you can see in the screen of your mp3 player showing "full" near the battery symbol.   

it might be locked

That's because u didn't plugged in your computer. Your computer have to be turned on..

you need to upgrade your usb port go on the samsung website it will tell u how

Have you tried resetting the player? get a thin paper clip or something similar and push the tip into the hole. then try charging it again untill fully charged, and remove from PC. Good luck