Hello sempra,

I just checked on archos website and they do not seem to offer any firmware download for the 45c platinum.

Their FAQ mentions the following as a reset procedure:

1. Turn off the device by using ON/OFF (remove then replace the battery if necessary)

2. Press simultaneously Volume + and the ON/OFF button until a green robot is displayed

3. Hold Volume + and press once the Home button on the font casing

Use the volume buttons to scroll down and up and the Menu button to validate.

 

If you failed to access to the recovery menu, please start again from the beginning.

 

1. Select "Wipe data/factory reset" and validate to restore your device to its default settings
2. Select "Yes -- delete all user data" and validate to remove all the user data
3. Select "Reboot system now" and validate to reboot the device

 

But it looks like this is the procedure you are following. Depending on what you flashed as a custom firmware, you may not be able to recover your device using that method.

I'd recommend that you contact archos support requesting the stock firmware for the 45c Platinum, it's probably best not to mention how you ended up with a not booting phone 

Feel free to share the results, it may be interesting for other users as well.

Cheers.