You probably pressed the delete button...
You can try a recovery program... no guarantees here. Google Android data recovery and you'll come up with several different free downloads.
or try Mobieldic Android data recovery app:
http://www.mobiledic.com/android-topic/recover-data-from-android-internal-memory.html
it's not free, but may works for you.
ok, thanks. I will install it and see whether it can recover the files or not.