iPhone shows unavailable

If you see the message that your iPhone is unavailable, then it must be restored to Factory Settings.

You can see how to do a Factory Restore without a computer here:

If you see an iPhone or iPad unavailable message or Security Lockout screen - Apple Support

If you want to do a Factory Restore from a computer, see here:

If you forgot your iPhone passcode - Apple Support

Then you will have to restore it from a computer. See the second link in my first answer.

No. Windows 7 is no longer supported.

You will need a PC with Windows 10 or 11 or a Mac.