After not using my Garmin GPS for two or three weeks, I tried to turn it on and nothing happened. Ends up the battery was dead. I first thought I must have a bad battery - at least the battery is removable. However, after searching online I found that a lot of people have the same problem.
When you turn a Garmin nuvi (at least the 855), it puts the device into sleep mode so it will start up quickly (providing it hasn't been shut down too long).
In order to really shut the device down you have to: [more]
Remove the battery, put it back in then hold the power button down when it's trying to boot up.
Hold the power button down (actually, it's pushing it to the side) for 8-10 seconds - but sometimes this works and sometimes it doesn't.