Bitcoin sent to a defunct address - what happens to it?
If the address is valid then the bitcoins will stay there until spent.  If you don't have the private key for that address they are now unspendable.

However, in general Bitcoin uses addresses in a different way to that which might be considered immediately obvious.  An address is generally considered a single-use item and as such it is entirely possible that your wallet contains the private key for the address to which you sent these coins, as well as any other addresses which you might have created.  Take a look at your wallet to see if it is present.