I deleted, added and changed some tags linked to a building and now the whole building is gone! How can that happen?
More importantly: how can I undo whatever change caused this?
The changeset is: 59606125
I had also done 59606511 shortly thereafter.
Hi. I checked your changeset and then the map. It looks like all of your edits are still there, but the building had lost its “building” tag, so it wasn’t displaying on the map as a building. I added the tag building=yes and saved the changes, so check again after it updates and see if the building is displayed.
It looks like someone has fixed it up a few minutes ago, after seeing your post.
In general, you can click on an object in your changeset, and see its current state. It may be deleted (this one isn’t.) At the bottom is a link to see the history. A great tool for tracking down what happened. 
I don’t know what you mean by “an object in your changeset”.
When I go to a changeset, I see the description I had entered, followed by a box containing five tags (changesets_count, created_by, host, imagery_used and locale), then a Discussion field and an Unsubscribe link.
Hi, below the Discussion Field box is a list of the objects edited in the changeset.
In the case of your changeset :-https://www.openstreetmap.org/changeset/59606125#map=19/45.52031/-73.57560&layers=N
There is one object edited, “Ways (1) 351760206, v4” click on that way link :- https://www.openstreetmap.org/way/351760206, it is a page showing the current state of that item.