Help with Ipod Syncing

so i recently rebuilt my computer and i reinstalled itunes and reput my music into it. i downloaded a new album and put the files into my itunes thing. so i log into my itunes account and make it all authorized and what not. so i hook up my ipod nano and it pops up a thing saying my ipods library is connected with a different computer or something like that, so it asks if i wanna erase the music and re sync, without thinking i check the box that says "dont ask me this again" and press cancel. now if i press sync manually it doesnt do anything. i cant put the new album on my ipod. how can i fix my problem /