The most common culprit is the battery.
Unfortunately if it is the battery, and probably if it's not, you'll have to reset it. Bike computers do fail so it's a good idea to keep an eye on your mileage so you know what to set it to when you have to reset it. I have looked up the circumference factor so many times I have them written down-- and I actually know WHERE it's written down. I also know where the instructions are, because I've had to dig those out again and again. Typically well over a year later when I have no clue what logical place I stored it last time.
I'm not overly concerned with my total mileage so just knowing the ballpark is good enough. I like to keep track of how many miles ago did I get a new chain, a new cassette, that sort of thing. It's fun to know that in a week or so I'll be hitting 9000 miles on this bike. But it's not terribly important so when the battery does go (like it did a few weeks ago), it's not a huge deal that my estimate was possibly up to 50 miles low or high.
2009 Trek 7.2FX WSD, brooks Champion Flyer S, commuter bike