sorry to hear about your loss - two of my jirds have developed tumors recently and i found one of them dead last week so know how you're feeling. I've had ongoing problems with my degus too, due to maloclusions of their rear teeth causing allsorts of problems - its cost a fortune in vets bills, with a minor op every 3 months.
my girlfriend had a degu on its own quite a few years back - unfortunatley due to bad advice from the pet shop (no mention of sugar free diets etc) it died at about 2 years old after developing cataracts and showing other symptoms of diabetes. however at the time she was able to give it a lot of attention and it seemed happy enough on its own. I've always had mine as a pair, and they do form very close bonds, my two degus spend hours grooming each other and almost always sleep stacked one on top of the other. however I think that you may have to be careful introducing any new degus to the one that you have - they can be territorial and you could well have problems just adding another straight into the cage. my two were litter mates so had no trouble but if you are planning to get another i'd definitely look into it first.
so sorry for your loss ! I don't know much about degus but do you know how old it was - perhaps it died of old age. Or heat exhaustion 
I think the average is around 5 year.
