It is probably the charging port. It seems to be a common issue with this model of phone.
If it is the charger port on th eD900, You have 3 options.
1. get it repaired, under warranty if possible
2. Get a new one
an external battery charger and charge battery separate from
phone.Another battery and charger will set you back about 10 dollars on
For more information and links to the parts check the following link below.
Samsung SGH-D900 will not charge problem