The router was set so that I could connect to it only via a specific IP.
By accident,I've changed the class of the IP on ether2 and, of course, I can't connect to the RB450G anymore.
What can I do?

Thank God the problem got solved without a reset.

Would the software reset have deleted my backup files from the NAND?

You could have just connected to the RB450 with a null modem cable via the serial port..