Wow thanks for all the answers (even the sarcastic ones)
I found the answer on the Garmin website:
Q. Why won't my Forerunner 205/305 turn on after charging?
A. If the Forerunner device has locked up during charging, it may need to be reset to power back on. To do this, please hold down the "Mode" and "Reset" buttons at the same time and then press the "Power" button to turn the Forerunner on.
Once the three buttons are released, the Forerunner should power on correctly.
Updating the unit's operating software is also recommended. Please download the WebUpdater software update tool to complete the software update.
If this key sequence or the software update do not resolve the lockup please call or email Garmin technical support for further assistance. Please click here for contact information.