how can i reset my phone if it has been locked

It depends upon what you mean by "locked":

There's multiple kinds of "locking" on devices. Click on the links below to find instructions on how to deal with your specific situation:

1) iCloud: Activation Lock - https://support.apple.com/HT201365 - This is a security feature so only legitimate (i.e., not stolen) redistribution of devices is possible. See: 'How to remove Activation Lock' --> https://support.apple.com/HT201441 - If an activation lock was put on a device, only the owner of the account that applied the lock can remove it. Get them to remove it or return the item for a refund. If you are the owner, see the part at the end about asking Apple to reset the device.

2) Device lock: "If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled" click here --> https://support.apple.com/HT204306

3) Carrier lock: Read this link on "How to unlock your iPhone for use with a different carrier" - http://support.apple.com/HT201328 - Only some carriers offer the ability to unlock iPhone for use on any carrier. Follow the instructions in this article to unlock your iPhone.

Click on this link to read what to do "If you forgot your Apple ID" --> https://support.apple.com/HT201354 Note that typically this is an email address you use.

Click on this link to read what to do "If you forgot your Apple ID password" --> https://support.apple.com/HT201487

or go directly to this link --> http://iforgot.apple.com/

Also read about "What to do after [or before] you change your Apple ID email address or password" - https://support.apple.com/HT204071

If you still need help:

Contact Apple for help with Apple ID account security "This article provides country-specific Apple Support contact information for customers seeking help with their Apple ID password or other security-related issues."

https://support.apple.com/HT204169