Disconnecting a power cable from an NTL modem can not cause your PC's username, password, DNS, POP3, SMTP settings or any other related items to change.
In fact, power-cycling the modem is often recommended as a good method of 'kick-starting' a cable connection.
As you appear to have internet access, but apparently not email access, it seems that you are currently a victim of NTL's email server upgrade process, whereby you lose connection to the email server for a while.
Be patient, it will return. Meanwhile, check out the server status pages at ntlworld.com