When I was younger I took piano lessons. For 4 or 5 years (from when I was in Grade 2 or 3 until I was in Grade 7). For years after that there was a piece or two I could sit down at Christmas and play.
Now I can still sit down and plunk out a melody. It comes in handy when wondering what a new tune sounds like. I had enough knowledge of theory to understand quickly how to know what note i was playing by counting frets on a guitar neck. I can still, with time and effort, sight read and identify notes on the clefs. But that, apparently, is about it.
For Christmas this year the whole family got an electric piano from Santa. Last weekend I went downstairs to get something out of the freezer and decided to sit down at the piano, just for fun.
With the instrument we got 2 books of music. One was a collection of classical pieces which are listed as beginner, intermediate and advanced pieces (although really even the beginner pieces assume a knowledge of what notes are where on the clef so not truly beginner). The other is a book of Christmas music.
I looked up one of the beginner pieces and was fine as long as I only tried one hand at a time. But both hands was not gonna happen with any sense of accuracy or consistent rhythm. Then I grabbed the Christmas book and turned to "What Child Is This" which is a piece I pretty much knew by heart 30 years ago. I believe that the arrangement in this book is very close to the same as the one I learned years ago. It is (as best I can remember) in the same key. And I would not play both hands together at all. I could barely play the melody line and the extra notes in the right hand with consistent rhythm. Both my music reading and my muscle memory of finding the right keys without looking have long since faded (and to be honest neither was great shakes when I was taking lessons as arguably I was less than consistent in practicing).
Apparently piano is not like riding a bicycle. Don't do it for a whilee and you can't jsut pick up where you left off.....