Okay. I wrote a measure which was quite complex but worked well. I moved on to soe mother measures and then noticed that this measure now had an exclaimation mark in front of it.
When I click on it it says "To use special characters in a measure name, enclose the entire name in brackets ([] and add a ] to any closing brackets in the name."
Helpful message with a but. With the message showing I cannot in anyway see the original expression to check this. I know I did not use special characters but without being able to access the expression I can do nothing to fix it. I have tried expanding the formula box but it is still empty, no way to edit.
I need this so I will now go and delete the original and rebuild the expression. I would be interested to know if there is a way to see the underlying code for future repairs though. In the power query editor you can get around this kind of challenge with the advanced editor. Any pointers would be gratefully received.