Hello, I appear to have shot myself in the foot with a nuclear missile.
I had a fairly complex (to me anyway) digital asset that I built up over the course of many hip files as the larger project I was using it in progressed. Specifically, I have a “panel” asset, that I was using to build an house for an animation. So I have “houseAnim_00.hip”, “houseAnim_01.hip”, etc - each with an evolving panel asset embedded in each file.

I wanted to re-use the panel asset in a new project so tried to make one independent of any hip file. Obviously I did it very wrong:
With an instance of a panel selected in the most recent hip file, I created a new asset with the same name (I know, I know, what was I thinking…), choosing to save it to library rather than the file. I get a message about some instances having spare parameters etc and select ‘no changes’.

Next I open a new hip file and add a panel from the tab menu. The asset is there but its completely broken - as in most of it - innards and parameters are missing. Think I saved it as the wrong node type.
Even worse, when I open any of the old files, they use the new broken asset rather than what I would assume is still embedded in those files. Some of the old nodes (presumably those that had spare parameters / where adjusted from the master) contain a full sub-net and parameters, but all the nodes are changed to blend nodes!

Can I recover my old working asset that was embedded in the hip files? Even if its a few versions old it would be a major relief.

Crisis averted. In case anyone has a similar panic in future:
All I had to do was remove the custom otl file in doccuments/houdini17.5/otls.
The hip files now find the embedded panel asset as before.
Obviously not great if you have a bunch of custom assets already, luckily I didn't.

Now to spend some time learning how hda's actually work.