When you're working with Lightroom's stacks feature, you should see small icons at the top right of each thumbnail when you're in the grid view in the Library module. They show three things: that the image is part of a stack, the total number of images in the stack, and where in the stack a particular image is.
They look like this:
Aside from their informational role, they also have a functional role. If you want to change the stack's cover image, you'll need to click on that icon.
But what if it's gone missing and you can't see them anymore?
The most likely reason they're no longer visible is that the option to display them is turned off. Here's how to fix that.
Go to View > View Options (or press CMD-J (Mac) or CTRL-J (Windows)).
In the popup options panel, make sure that the Grid View tab is selected, then check the box next to Show Grid Extras.
In the Cell Icons section, check the option for Thumbnail Badges.