The password for a virtual machine can be easily and quickly retrieved if you stored the key pair you associated with the machine on the portal. To retrieve it, click the 'Show Password' link on the virtual machine details page.
If you did not store your key pair on the portal, you can still retrieve the password by supplying the private key with which the virtual machine was created. In this case, when you select the "Show Password" button, another dialog box will appear, prompting you to enter your private key. Paste your key into the dialog box as shown and click on the "Decrypt Password" button.