Purchased songs no longer appear in the purchased

Yes, iTunes some times forgets that you have bought the songs you have.  If you go into 'Account Information' in the iTunes store by clicking where it shows your email address on the top right button; you should be able to access you actual 'Purchase History' This will display all the songs you have ever bought on that account together with their order numbers.
Incidentally I have found that the 'Purchased' playlist can often sort itself out after you make another purchase. I guess it just needs to sync up with the store computer to jog its memory!