Excel freezes after Userform opens

Good Morning All

The attached workbook only has 1 page with a button to open the Userform. The Userform only has an Exit command button.....

When the Userform opens, excel freezes and won't close the Userform either by the button or the red X..

I have never come across this before and I have obviously done something....

Would anyone know what is making it do this

Many thanks

Jim