Your captions are still there. The project media section has two view modes, and the caption is printed under the image in only one of them.
Full-width prints the caption under each image. Grid does not print anything under the tiles. Switching to Grid therefore hides every caption on the page at once, which reads exactly like the captions having been wiped.
Switch back with the toggle beside the Project Media heading and they all return.
In Grid view, open the image to read its caption
The caption is not unreachable in Grid, only not printed on the tile. Hover an image and select the frame button, View in gallery. The full-screen viewer opens, showing the image title across the top and the caption underneath the picture.
Long captions are shortened there. Anything past 100 characters is cut with a Show more control that expands it in place.
Phones are always in Full-width
On a screen 768 pixels wide or narrower, the gallery switches itself to Full-width and stays there. So on a phone you always see captions and never see the grid, and the gallery viewer's frame button is not offered. If a caption is visible on your phone but not on your laptop, this is why, and the fix on the laptop is the view toggle.
Nothing you did removed them
Captions live on the image file rather than on the project's text, so they are not affected by editing, re-publishing or restoring an older version of the project. If a caption is genuinely missing in Full-width view, it was never saved on that file, or it was saved by a different account.