When you "just left the bike", as written in your first post, it won't end the rental period. It may even be that someone took it when finding an OV-fiets with the key still inside.
What @Thom is writing about the 10 euro surcharge only applies when the bike is returned properly at Muiderpoort. So the key should be put in the key vault after choosing the option to return a bike. Just opening the gate at the entrance to the bike shed doesn't end or begin a rental period! And as an email was sent as a reminder, the rental period clearly wasn't stopped properly.
As you're not in Amsterdam it's best to contact customer service immediately and have the rent ended manually. Explain to them where you left the bike, but keep in mind they'll always go and check that the bike is actually there. When it's not, you'll get a fine for stealing a bike as when getting an OV-fiets subscription you agreed to being responsible for the bike until the rental period is properly ended.