From
http://www.ipaqrepair.co.uk/ipaq_battery.php
2. How do I hard reset the iPAQ 5450, 5550, 5555?
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 ten seconds the soft reset button this is located on the lower right edge of the unit. The screen should reset and the iPAQ will boot up again.