my sony nwz-s545 mp3 player won't charge?

I have a sony nwz-s545 mp3 player. It worked perfectly until last week. Now I connect it to the computer, it says "Connected USB. The timer and alarm will become invalid", stays like this for 10-20 seconds, but then turns off, and will not charge. Resetting the unit does not make a difference. Could this be a problem with the battery ? How can I check or replace it ?

YY
March 2011