If I have locked my stolen iPhone, it cannot be accessed right?

Was the phone locked?

Did the thieves know, surreptitiously learn, or trick you into

• Revealing your pass code?
• Revealing your Apple ID and Apple ID password?
• Removing the phone from the list of devices associated with your Apple ID? (Beware of "phishing" by criminals pretending to be "Apple Security" or some such, who claim to want to reunite you with your phone, but actually want to fool you ihelping them "make your phone their own")

If it was passcode locked, and you had Find My turned on, and you made a request to mark the phone as Lost, and the thieves don't have any of your credentials, and it is still associated with your Apple ID, then they may be a little frustrated around now.

P.S. - One scam that thieves sometimes pull is to contact a phone's owner and claim that they have access to all of the phone's information, and will sell it to the "dark Web" … except that they really want to "help you" and they will erase your phone without disclosing your data if only you remove your phone from your Apple ID account (which of course will help them "make your phone their own").

This is a big scam. They are using a blackmail threat (exposing your information) at the same time they're claiming to be your "friend"– so there is no way they are your "friend", and no way you can trust them. Probably they do not have your data at all, but are hoping that you will think that they do.

What does “iCloud locked” mean?

See this article.

If your iPhone or iPad is lost or stolen - Apple Support