First can you follow this link to recover your iTunes password http://support.apple.com/kb/HT1911
Then once you ahve your password XXXXX you contact iTunes to re download your songs to the new mahcine.
Basically iTunes allow you to request a redownload of your purchased items one time. You will need to contact iTunes support by emailing them by filling in this form http://www.apple.com/support/itunes/contact.html?form=download&topic=Downloading%20iTunes%20Store%20Purchases&subtopic=Download%20errors
Once they receive your email they should allow your account a one time download licence for all your content.
Let us know if you need more info.