Change the battery. It is a 9v battery behind the small door on the back of the display panel.

There are two batteries. The 9V behind the display may maintain some memory. There's a bigger 6V battery down in the bike that is kept charged by pedaling - that's why there's no cord. Pedal like crazy for a minute or two and even a fairly dead 6V battery will probably recharge enough to turn on the display. A full recharge will take about 20 minutes of pedaling, and need that much every week or so to keep topped off. If it dies fairly quick, get a new battery. About 20 bucks if you look online. The original equipment battery number is 0017-00003-0685. (Just found this out after picking a HR 5500 up at the flea market. Fortunately my son pedaled hard enough to get the display working after I'd replaced the 9V and thought the display was shot. Batteries Plus lists them in stock, as do lots of online places, though the shipping can be kind of steep in some places.