Scratch

That problem happens a lot. Can you see the sprite on the stage? If you can, click on the copy tool near the top of the stage, and copy the sprite. The copy will appear in the space with the sprites, then you can move it out of the way and delete the one that didn't show up.

If you can't find the sprite on the stage, you have another option, but it can be annoying. Save the project and open a new, blank one. Delete the scratch cat sprite and click on "extras" at the top, then select "import project". Select the project with the invisible sprite and import it. That should make the sprite re-appear, but importing reverses the order of the sprites and moves them out of the place they were. You might just want to export the sprite and import it back into the project that made the sprite disappear.