Someone accidentally deleted a key which had loads of data from firebase realtime db via the cloud console.
No backup is available, so is there a way I can recover that?
PS: Note that Its not a whole project or a database that's been deleted, but just a key from firebase that had lots of data!
If you are a blaze plan user, you have the option to set up automated backups for your database.
However, seeing that you are asking this question I doubt that you have those available: AFAIK, there is no way of restoring deleted data apart from restoring a backup since the related data has been removed from the database.
There are great concepts and options for database backups as well as database auditing / versioning out there, I recommend looking into that to prevent further incidents.
EDIT
As Joseph pointed out in the question comments, you can contact firebase support to help you as they keep backups of your data: https://stackoverflow.com/a/35959363/2305594
EDIT 2
As you pointed out, you did specify 'not the whole database'. However, you could download the backup and extract the specific data yourself, manually adding it back to the database. The backups are stored on a google cloud storage bucket in an easily readable JSON format.
