Buy a new one? Or if you cannot afford a new one try YouTube and mend it yourself, maybe the leg screws were loose and the activity made it fall a part. There are many options, just try not to panic and think about your possibilities, the truth is always an option.

You should tell them the truth and offer to repair/replace what you broke. You have a relationship with these people and if you base it in lies it won't have any strength when you encounter an actual problem. If they're cool they'll think the whole thing is funny and respect you for being honest. If they aren't cool, better to find out now rather than wait until the they explode over something else down the road. Though I'm assuming you are an adult. if you're a child and this is going to get someone shipped off to boarding school or kicked out of their foster home maybe keep things on the downlow.

Fix the bed. What are you a child