Internal hub gear suddenly stopped shifting I have never had an internal hub gear bike before, so I don't know much about them.

My issue is that all of the gears stopped working suddenly when I was just riding the bike. The bike has 5 gears, everything was working fine, but one time when I was riding, the gears suddenly just broke. Now, when I change gears on the handlebar, nothing happens with the actual gears, it just stays in the same gear. 

What could be the issue, and how to fix it?
Almost certainly the gear cable has come off the hub at the point where it's adjustable. Without knowing which make and model of hub you have I can't be more specific than that. If the bike is new take it back and get them to show you how to fix it, or at the very least fix it for you.

If not, work out what hub you have (the manufacturer plus five speed will probably be enough) and search for a diagram of how it's supposed to be set up - probably if you find a photo of the rear wheel/hub area from the right hand side that will be enough, as you'll be able to see how the shifter mechanism attaches.

If you can edit your question to have more details, or include a photo showing that area, we will probably be able to make more suggestions.