:S I don't even know what you've changed and what the problem is
The domain shouldn't matter much unless your computer credentials is managed by the network (which isn't the case if this is a single computer you use at home)
Now, the account you're trying to log in to, if you're sure you got the username right and the password right, there's a possibility that when you were changing the password you had something like Caps Lock on. So try turning that on and typing your password. Also, if this is a laptop, a turned on Num Lock could have been the issue too, so turn Num lock on and try typing the password in.
If none of that helps you can try to see if the Admin account still uses no password. To access it, restart your computer and during bootup repeatedly hit F8 key, until a menu shows up with choices. Use keyboard arrow keys to go up or down and select Safe Mode. once it loads up, there should be an Administrator account available. If you're using the network login style, just type in Administrator for user name and leave password blank. try logging in that way.
if it works, you should be able to go to control panel and manage your other account that you use regularly. If not, ouch, but there's little solutions from there.
You could try 3rd party password unlockers, but I don't know of any that I'd recommend.