I made this geometry with many edits. I went on to edit other geo node & when I came back to the previous node, every single node except the last one was vanished.
I tried Shift+E but nodes are not hidden.
Geometry is visible & accessible but all the nodes are gone.
If I click on the lock button in the last node, this message “ The operation will discard locked changes!” appears and if I hit enter, geometry gets deleted.
I tried restarting & reinstalling houdini but nothing is working.
I've attached screenshots below.
Please help.

seems like you have deleted the history RMB in viewport/Geometry/Delete History, that will lock the geo and remove all upstream nodes
while there is usually no reason to delete history as you can lock the last node, cache to disk or use Stash SOP, once you delete the History and save/reopen the file, there is no way to bring the nodes back as they were simply deleted

Since all the steps to create the geometry was deleted, for safety, I would click on the node and save as bgeo or obj. At least you won't loose your model, though you lost the steps. I would look through your backups, though, so see if you can find a model that had the steps.