Looking at the first picture you will see a dislodged part in the piano. Take note of the red block of felt in the circled area. This is the top of your jumping jack.
Look at the second picture to see it glued back into place. This loose part we call a "jack" and it drives the hammer to the string.
A quick 8 minute fix if you have good access to the parts and great mechanical dexterity. (Remember to re-seat the jack spring)
This is occasionally a problem with older pianos where hide glue was used in the parts. If you look into your piano and the wooden parts are a similar dark color you may want to check the glue joints for looseness. Often you will hear a click or rustling sound when the notes are played, a classic indication of a loose glue joint or part.