Translation:There existed only one key for the room.
The verb at findes (past tense fandtes) means "to exist".
In theory it could also be a passive of at finde (= to find), but I've never heard it used like that. The passive of at finde is always at blive fundet.
'There existed JUST one key for the room' is not accepted yet. Hopefully it will in the future as I consider it equally correct. ( Although a terrible English sentence)
"Only a key for the room was found there" was my answer based on the earlier learning. I get Viktor's point but must agree with Yorifik, it should "only a key for the room existed there" to make any sense in English. I would suggest a rethink on this question.