Well, you could always re-rip from your CDs. But it could take a week.
There are data recovery services that can find files there were deleted. They can have success if (big if) you have not used the drive after the files were deleted.
What happens when you delete a file is the space the file used to take is given back to a pool of free space, to be re-used. when you write a new file space for the data is taken from this pool. So if you have not written new data to the drive all those files are still there on the drive but bt the index folders are gone and it will take a data recovery expert some time but he should be able to find your stuff. If you have not yet used the drive. It might cost you between $350 and $1,000 to have some on do this. But if the other option is to buy 15,000 tracks from iTunes or spend a week re-ripping CD spend the bucks.
The other option is for you to buy some data recovery software and try to do the job yourself. But if you are here asking these questions you are not quite qualified to be your own "data recovery expert". Pay a pro.
Why in the world would you not have multiple backups? You should have three backup copies with some stored in an off site location.