Dialogue box does not appear after attempt to place digital signature.

[Redline Building Design]

I just converted a MS Word Doc to a PDF and was trying to place a Digital Signature, like I always do.
However, after placing the Signature Box, the Adobe Acrobat DC window just went grey and there was no dialogue box to confirm the signature that I wanted to use and enter my password.

[Redline Building Design]

After numerous attempts and with different files, having to stop the program using the Task Manager, I eventually did the inevitable "turn the computer off and turn it on again." >>> Fixed
But, why did this happen in the first place?