Oddly the password works in the the command line.
I've created a new account on command line and logged to the the desktop from that. However the password of my old account no longer logs to the desktop.
Your old account is likely gone. I wouldn't expect reinstalling an OS to preserve the accounts (although your data may still be there in /home/YourOldUsername/. You can confirm this yourself from the terminal by issuing the command cut -d: -f1 /etc/passwd |grep "YourOldUSername" where YourOldUsername is, you guessed it, your old username. The absence of YourOldUSername confirms that the account is gone and you can check to see if you still have data in /home/YourOldUsername/ by starting nautilus or your favorite file manager with elevated privileges (as your new account may not have access to those files) or simply take a look via the command line with something like sudo ls /home/YourOldUsername/ If you see Desktop, Document folders, etc. you can drill down and find your files. If the problem is a forgotten password see this
Sources: How do I reset a lost administrative password?
A command to list all users? And how to add, delete, modify users?