Formal:
* C'est verrouillé. (pronounced: Say ver-roo-yay) - This is the most literal translation and the most formal way to say it.
* Il est fermé à clé. (pronounced: eel eh fehr-may ah clay) - This translates to "It is locked with a key."
Informal:
* C'est fermé. (pronounced: Say fehr-may) - This simply means "It's closed."
* C'est barré. (pronounced: Say bah-ray) - This means "It's barred."
The best way to say it depends on the situation and the level of formality you want to use.