Installing 2200mah battery in iRiver iHP120?

Unit taken apart, new battery plugged in, charged using a 5v-1.2 amp charger, and whilst charging some 15Gb of songs were copied to a Dell computer (2.4 ghz-WIN98SE). After five hours the unit was disconnected from the charger and the Dell, and the screen message remained as 'Connected'. The reset button was pressed and the iRiver logo page appeared with the red indicator light on. The unit was plugged into the charger and the green light also appeared, but the screen still remained frozen on the iRiver logo. After the charger was again disconnected the screen and red lamp remain lit until fading. When plugged into the Dell the unit is not recognised. Does anyone have an idea as to what has happened, and what might be done?