The form would be förlorade, since the keys are plural, but it still sounds very odd to say that about keys. Vi är förlorade works for 'We are lost' (not as in 'lost in the woods', but as in 'our lives are ruined'). It's the participle that makes it too weird, because as a verb it's fine: jag har förlorat nycklarna. If you really want to use a participle, borttappade works though. That's formed from a particle verb tappa bort which means 'drop', 'lose'.