When you create a Write.as account, you can add either a password or an email address, or both, so you can log in again in the future.
If you have an email address on your account, you can log in without a password. Simply go to the editor, hover over Anonymous at the top of the page, then select Existing blog. Enter your username only, leaving the password field blank, and press Log in.
You'll receive an email with a link that'll log you in to Write.as. Be sure to check your spam folder if you don't see an email within a minute (and please mark it "Not Spam" if it's there), and be sure to click the link within 15 minutes — otherwise the link won't work and you'll need to log in again.
To log in with your password, simply visit the login page and enter your username and password, then press Log in.