There can be various reasons why your Keys are missing.
It might be because you have pressed 'Next' on the Wordy Weekend event. This consumes keys to skip the character.
It also requires Keys to use the ‘Save Me’ option and that the cost doubles every time you use it. For example, the first time you use the feature it costs 1 Key, second time 2 Keys, third time 4, and so on.
It could also be that you have accidentally purchased something in the game requiring Keys. If you didn’t purchase anything someone else might have access to your device (or another device using the same Online Save account).
We cannot reverse purchases made with Keys. We recommend that you are cautious before you click on buttons with the ‘Key’ icon.