from http://www.ipaqrepair.co.uk/ipaq_battery.php
 

5. How do I hard reset the iPAQ 3950, 3955, 3970, 3975?

Just hold down the calendar and today buttons, these are the two outer buttons on the lower left and lower right, then using a stylus press and hold for at least five seconds the soft reset button this is located on the lower right edge of the unit. The screen should reset you will now need to press the soft reset button again to power on the unit as the power button will not do this.