If the wiring is not damaged inside the drawer and the latch mechanism is working properly (solenoid free to move etc), then you may have either a faulty solenoid or a fault in the drive electronics. Make sure it still doesn't open when you print daily reports and that will prove that it is not a programming issue.
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If anyone needs more information, the Sharp XE-A303 user and programming manual is here:
http://the-checkout-tech.com/manuals/cashregisters/Sharp_XE-A303_operation_ENG_ESP_GER_FR_DUTCH_manual.html
Registerman, May 2011