youre probably just not signed in as your user.
things that users have purchased from the store, can only be used by that particular user (i believe). im also pretty sure, that when you remove, then replace the 360's hard drive, it wont automatically sign you in as the user. just make sure that your user is signed in (to xbox live as well, just to be safe) and it should work.
if they are somehow not on the hard drive anymore (you didnt insert or disconnect the hard drives while the system was on and running did you? in that case, you got what you deserved) then you can easily re-download anything you've purchased from the store.