This is network specific in regards to your nick registration, not a connection issue.
The password might be case sensitive, in which case a password of pAsSw0rD is not the same as passw0rd
Be careful when entering the password that the case matches what you used when you set it.
If this doesn't work, then you might try connecting with a non-registered nick, and checking the nickserv options to see if there's an option to have your password sent to you.
I know this is available on some networks.
If this option isn't available, then you could try contacting an IRCop or network administrator to see if they can reset your password or remove your nick registration.
If they can reset it, then you can have them set it to the password you think it should be.
If they can't reset the password, but can remove the registration, then you'll have to go through the process of registering your nick again.
Note: These last items may require some kind of proof that you are the proper owner of the nick.