This is an easy fix, and built right into the JSE.
Go to the JSE website at https://jse.vic.gov.au/ and click on Forgot username / password
The form will change:
Enter your email address in the space provided, and click on Submit.
If all goes well, you'll get an email in a minute or less with your username in it, and a link to set a new password. Click the link and you're back in! Whew.
Wait, it didn't go well!
Sorry about that. The most likely thing to happen is that the system doesn't know your email address for whatever reason.
Has your department or agency recently been renamed? Try with the old version of your email address - if IT is forwarding your old emails to your new address, this could be a handy way to get back in to the JSE.
Handy tips for the future
To give yourself another way into the JSE, consider adding a personal email address to your JSE account. You can then use that to regain access to your JSE account if your main VPS address has changed. Don't forget to update your VPS address once you're back in again.