Extract this file with winrar and put it on a blank new SD card (2gb or less) in the Tomtom. Then try a pin reset and turn the Tomtom on.
If the Tomtom still doesn't start up, then it's probably a hardware issue. Even if you learned how to open the GO 300 and repair it, you'll have a very hard time finding replacement parts.
You should check out the new models, it's probably not worth a repair. The GO520 does everything a GO 300 does and should be pretty cheap. Even the ONE Classic will do everything a 300 does except GPRS PLUS services.
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