This is my wallpaper at the moment, display res set to Native (1366x768). The image is originally square by the way. So I deliberately have this image in the centre of my screen, with black bands either side.
Now I have changed the resolution to 1024x768. The whole available display does not fill the physical screen.I have set the Wallpaper to "stretch" and it fills the whole available display, though as I've just said, not the whole physical screen. You can see that the image is stretched laterally.
It's possible then, that the person who "fixed" the OP's computer, caused the display settings to change from the previously set resolution to something different, and also caused the Wallpaper image's aspect ratio to be different to what it was before.
In fact, IMHO the symptoms described suggest that the resolution has been increased in the course of the "fix", and whatever Wallpaper was previously confined to a more "stocky" aspect ratio has now been "laterally stretched" by virtue of the display being set at the native res. If so, I am recommending the OP retains the native res, as in the long run it will be better for performance and probably reduce the risk of undetected chronic eye strain.
I agree - more context is absolutely needed to avoid further idle speculation!
bb