After a 10 hour drive home from my parent's place in Florida, I woke up Foxie cat and she started crying and walking in circles, bumping into things. The pet-sitter's report from the day before was normal, but at some point in the night or day she had gone completely blind. We found a veterinary emergency room. After $1000 and only five hours of sleep we found out that she has mild kidney dysfunction, which caused her blood pressure to soar, which caused her retinas to partially detach.
There is some hope that with medication she'll regain her sight, but
in the meantime the poor thing is completely blind, and had already gone
mostly deaf. She's 17 years old, and in fairly good shape for all
that. We're assured by many people that (indoor) cats can function just
fine by sense of smell and navigate by memory if you're careful not to
rearrange rooms. Her quality of life will be fairly good (other than
when I'm trying to get her to swallow her pills). But it means we have
maybe 2 years left with her. Not the best ending to a vacation, but it
definitely could have been worse.
If you have an older cat, make sure that your vet checks blood pressure at the annual checkup. It can have many underlying causes, but it is the most common cause for sudden blindness in cats and can be controlled with medication.