A sad goodbye

Question
Last year i bough 3 female rats from my local pet shop, Over the year ive grown to love them so much each having there own character and cheeky ways. I never knew how much fun and joy they bring to your life, just waking up in the mornings and all three there waiting to greet you.

Sadly i found one of my females (monica) had past away over night, but i couldnt understand why, the day before she was her normal self, drinking and eating fine and playing with the other two. There was blood on the wall around there cage and blood in her mouth and front paws, ive searched the internet for some sort of answers but none seems to relate to her death. So i was just wondering if you had any answers, as im worried about the life of my other two rats and if there anything i can do to stop this happening again.

Answer
 There really are a number of things that could have gone wrong and it is impossible for me to tell you exactly what happened.  It could have been a stroke, heart attack, or head injury from a fall or trauma from one of the other rats or a toy in the cage.  If she was healthy before she died it's unlikely that the other rats are at risk, though if you are concerned you may want to have a vet do a necropsy (animal autopsy) on her to see if he can see what was wrong.  I'm very sorry to hear about your rat!