unlock the Message Box ?

[Richmond]

Today, whilst programming at the school one of the kids
wanted to close a stack, while keeping another open, and was unable to
because he received this message:

. . . until you unlock the Message Box

1. How does one lock the Message Box?

2. How does one unlock the MB?

3. How did this happen without any obvious
actions on either my behalf or that of my pupil?

Richmond.