How and why does a thread get locked?

As a relative newbie to this community, I am wondering how and why threads get locked.

 

I've been following the "Federal return rejection code F8960-019-04..." thread for some time and I noticed it's now locked.  Really sorry to see that.  I was getting a lot of good information from that thread and I am wondering how and when we will be notified when this error is resolved.