OK - “suddenly stopped working” is basically a typical exaggeration, often heard from a frustrated user (me).
When trying to add a new (valid, working, in-use) SSH key to Pageant, I was getting this error:
Couldn’t load private key (not a PuTTY SSH-2 private key
Except that it totally is a PuTTY SSH-2 private key, which I am happily using to connect to a remote server using PuTTY.
And my Pageant program already has some other PuTTY SSH-2 private keys which it previously loaded OK.
I had not, in fact, used this specific Pageant program in quite some time - but I had, in the interim, upgraded PuTTY.
My PuTTY version is:
Release 0.79 (the current latest release).
My Pageant version was:
0.62 (definitely not the latest release)
I upgraded Pageant to version 0.79, along with the entire set of PuTTY utilities - see here.
That fixed my error.
You are using the latest version of PuTTY, right?
You have upgraded all of the PuTTY suite of tools - and then remembered to actually use them, right? Which is what I had failed to do.