I've been working on this project for over a year and I have never had any substantial problem. Two days ago I was working on the project and everything worked fine. Today, when I try to load the project, I get the following:


[![enter image description here][1]][1]

The layer in question depicts points on my map. If I click on "Keep Unavailable Layers" and hover my mouse over the layer in question in the layers pane, I get the following message: `Unavailable layer! Layer data source could not be found. Click to select a new data source.`

[![enter image description here][2]][2]

I click and select the exact same data source, i.e. I "remind" QGIS where the .csv file is located, but then almost nothing seems to happen, apart from the layer's symbol changing from three dots to a cute cinderblock. The points on my map do not come back. 
[![enter image description here][3]][3]
I am certain that neither the .qgs project file nor the .csv have been moved. Nor have I updated my operating system.
The problem surfaced with QGIS 3.10 running on MacOS 11.1 (Big Sur), but I have tried with QGIS 3.16 too and I get the same error. The source file is readable by QGIS, because I can create a new layer with the same csv file and it works.

Why is this happening? How can I get my layer back?

Edit: If I try to copy the style (Styles > Copy Style > All styles categories) of the cinderblock layer to a newly created layer with the same source file (Styles > Paste style > All styles), I get the following error:

[![enter image description here][4]][4]

But if I copy the style before the old layer becomes a cinderblock (i.e. before clicking "click here to select a new data source"), I do get back my old points...

[![enter image description here][5]][5]

...but I don't get back the lines between the labels and the points. Before the problem, it looked like this:

[![enter image description here][6]][6]


  [1]: https://i.sstatic.net/TboRz.png
  [2]: https://i.sstatic.net/u9NS5.png
  [3]: https://i.sstatic.net/OXHKE.png
  [4]: https://i.sstatic.net/5Cbxr.png
  [5]: https://i.sstatic.net/h3cur.png
  [6]: https://i.sstatic.net/1DDM8.png