Probably the batteries have failed.
Start by pressing the slider and removing the hidden key.
You can use a screw driver or your nail - but lift up from the tab in the green circle and it should pry open, on some you might have to pull harder but don't worry you wont break anything. DO NOT USE THE TAB IN THE RED CIRCLE that tab is where the key goes and has a bottom you could break plus you won't get any where pulling from there.
Once the cover starts coming off just pull off.
The rubber buttons just slip right off to expose the battery.
The battery type you need is CR 2032 they are $3-$5 at local stores. When your putting the battery back - be sure to slide it in carefully to avoid damaging the contacts.
Place the rubber buttons back on and snap the cover back on and you are good to go.