If you are on the "Unavailable" screen and you do not see a Forgot Passcode option, your only option at this point is to use a Mac or PC to Restore the iPhone.
That will erase the phone and install the current most up to date iOS version on the phone, but you will still need your Apple ID (now called your Apple Account) and the Apple Account password to set up the phone again.
If you’ve forgotten your iPhone passcode or your iPhone is disabled – Apple Support
An Apple Store / Genius Bar or authorized Apple service shop will not normally charge you to simply erase the phone, and you can then set up the phone again from the "Hello" screen.
However, if there are other issues, then a service charge will likely apply.
Have you been paying your carrier for service even though you have not been using the phone for some time? If not, you may need to get with your carrier to get service established again. The carrier may impose a new lock on the phone, requiring you to complete some months of service before the phone will be unlocked.