Key action is a pretty simple mechanical process. My guess is that something broke off, or became dislodged inside, and it's interfering with the fulcrum-like action . It's labor intensive, but not hard to open up the kb, to look inside. LOTS of screws. Try this first:
On a very padded surface (like a bed, or couch cushion), gently tap the kb, on it's long side a few times. See if you hear any rattling noises when you flip it around. That'll confirm my triage. Good luck!