liamboyle06: log into the email management screen at
https://autoreg.autoregister.net/cgi-bin/xtrmail
using the username that works and check that the other username is still listed
If it is there try going to the password update screen at
https://autoreg.autoregister.net/cgi-bin/chgpwd and doing a null update (enter the current password three times) to refresh the account.
TezmitcH: If you log into webmail, are there any large emails in your inbox? If not, are you running any antivirus packages like mcafee, norton or netguard and does the problem persist if you turn them off?