Am I liable?
Went in and found one of the fixtures in the bathroom broken and most of the unit flooded.
You almost certainly will not be legally liable.
For you to be legally liable, a claiming party has to establish your negligence. That can be hard to do with spontaneous plumbing failure situations. But it is what they have to do. Eg they’d have to establish that you should have reasonably known of the situation before it spontaneously happened. Ownership of something is, alone, not enough to establish liability.
If you are not liable it’s up to you whether you pay for the other lot’s damage or not. It might be good manners and neighbourly to do so. If you have access to insurance that covers your legal liability, your insurer would almost certainly refuse your claim because they too would doubt your liability. You could check that if you do have such insurance.