Well, we can help, but you’ll have to tell us exactly what steps you took while adding that other IP address
Normally, adding a new address wouldn’t cause an old one to stop working. And it certainly shouldn’t prevent your password from working.
So it sounds like something unusual is going on
Also, what distro/version are you using? And do you have access to the hardware? That is, if you can’t remotely access your server, the only way to fix it is logging in at the console.
An alternative is that if you’re using Virtualmin, and you’re looking to setup one particular Virtual Server with that new IP address, you could go into Server Configuration -> Change IP Address.