Hide & protect sheet

You need to tell you macro which state your form is in. The trouble is that because the macro is triggered when you select the sheet (either unhide it, or select it), then it is always in the same state as 'unhidden', so you can't use this property as a test. You need to create a new test for this state prior to running the macro. One way to do this is declare a static variable globally up front, then change the state in your code as you change the window state.

Add this line outside of any existing procedure in any module

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add this to the workbook sheet module

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Change the code on the sheet 1 module to

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After every line of code where you have hidden/unhidden the sheet, you need to add another line that updates your variable

either MySheetHidden = True or MySheetHidden = False

You will need to debug this to make sure it works, as I haven't tested it

Thank you for the replay Mallycat!

At first I had some trouble inserting all the right code in the right place after your suggestion.
So...
this goes to the "Sheet1(Sheet1)" code

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this goes to the "ThisWorkbook" code

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and

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in the form, wherever changes are being made to the state of Sheet1 (hidden or unhidden).

I've spotted one small problem with this code: you were supposed to close the file to get the Boolean variable MySheetHidden to update.
In fact, if you were hiding Sheet1 and not trying to close the file, when you would unhide it, the form wouldn't pop-up. So I had to link the variable update to one more event and because of (or thanks to) the fact that when you hide a Sheet, Excel automatically selects the next Sheet, all I had to do was to add 3 lines to the "Sheet2(Sheet2)" code:

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So that's the smart macro that I was looking for and right now I personally see no flaw in it.