D900 won't turn on , But battery is ok

Your charging port may have broken. I am seeing more and more of this type of problem. Did you try a different charger? If it is the charge port, You have 3 options when the charge port breaks.

1. Get it repaired, under Warranty if possible. About 60 dollars
2. Get a new one, runs sbout 75 dollars
3.Get an external battery charger and charge battery separate from phone.Another battery and charger will set you back about 10 dollars on ebay.

For more information and links to the parts check the following link below.

Samsung SGH-D900 not charging problem repairs