Hmm, of course there are various approaches to how you might figure things out -- some old-fashioned "detective work", retracing all your steps and trying to figure out what you may have done and Picasa may have done. Or, you can take the straightforward ol-fashioned approach and use the Search function in your Explorer file browser -- if you had given the images descriptive names that would help, otherwise you are stuck with looking at everything with the proper file extension.
Or, hey, that possibility of the shots still being on the card is a nice optimistic thought!
But if nothing works, hmm...
What happens when you open Picasa? Does it automatically go to the images you actually have?
And, what have you done so far to "find" the missing images? Having more details will help us to help you!