Eye need help

I was at work all day, and when I came home, no more than a half hour ago, I noticed that my boston, Riza, had her right eye always closed while her left was always open. My sister was home, but doesn't seem to know what happened. We have two other dogs: my sister's boston, who is a little bit bigger than Riza, but they get along fine, and a boxer who's probably too old to do any damage (she's 12).

Riza won't let me open her eye to get a good look at it, so I can't really say what's going on. If anyone has any ideas, I would be so grateful. Thank you.