If flash was formatted and then the router restarted, then the router does not have an image to run. I would assume that it is booting to rommon. The only way to access the router at this point is through its console connection.
I would suggest that the best way to recover from this would be to connect a PC running a TFTP server to the LAN port of the router. Have a copy of the image for the router available to the TFTP server. Then through the console connection to the router configure and execute TFTPDNLD on the router. This will access the TFTP server and load a copy of the image into flash.
When doing a console connection, the terminal window only produces nonsense characters. How can I boot the router to produce the "rommon" or "boot" prompt?
If you connect on the console and get nonsense characters it is usually a sign that the speed of the console has been changed (or that your terminal emulator is not set for the default speed of 9600). I would suggest that you try various speeds in your terminal emulator. I would start at the bottom with 1200 and work my way up until you find the speed that does work. Note that with some terminal emulators like Hyperterm you may need to stop and re-start the connection to get it to recognize the change in speed.
Francisco is right. Use the "tftpdnld" command is the way to go ... unless you want to use the slow method zmodem.
Do you have another 3825 nearby where you can borrow the flash card?