To get a prompt back, you could type Control-C (often written as Ctrl-C or ^C):

^C
$

You get a new bash prompt, but the xeyes window disappeared. In fact, the entire process has been killed. Instead of killing it with Control-C, we could have just stopped it with Control-Z:

$ xeyes -center red
^Z
[1]+  Stopped                 xeyes -center red
$

This time you get a new bash prompt, and the xeyes windows stays up. If you play with it a bit, however, you will notice that the eyeballs are frozen in place. If the xeyes window gets covered by another window and then uncovered again, you will see that it doesn't even redraw the eyes at all. The process isn't doing anything. It is, in fact, "Stopped".