Christo de Klerk
Here is some weirdness that I will try to describe in some detail. When this happens, it kills speech from NVDA, but the computer is not frozen. If you give the Ctrl+Alt+n command, you hear the NVDA startup sound and it comes back. You do not get the NVDA shutdown sound. Here is what seems to be the cause and how I worked around it:
I am using Thunderbird as my email client. The columns in my inbox are set up to show attachments first (if any), followed by From, followed by Subject. As I have been doing email, every now and then I lost all speech and had to restart NVDA, just for it to die again when I got to a particular message. I used Narrator to figure out what the offending message header was and saw that in the From column,it showed "_Congrat_" and nothing in the Subject column, obviously some spam or malware kind of message. I then set up a rule to scan for the string "-Congrat" in the From, To, or Subject fields and to delete such a message. That seems to have worked around the problem.
I have no idea why this causes NVDA to go quiet on me, because having the offending text string in my message does not have any negative effect.