MacBook Air loads to a black screen with a gray lock

Is this the symbol and screen that you are booting into? See image below-

If this is the case then your MacBook Air is asking you to input a Firmware Password that either you or the previous owner of the computer had set. There is no software way around this. You will need to have the password to go further.

If, for any reason, you do not have or forgot that Firmware Password then you can try taking the MacBook Air to the Apple Store or an Authorized Service Center and see if they can help after you show them the proof that you own the computer. Not sure whether they can or will help, at least they will tell you what will be needed to unlock the MacBook Air.

Axel F.