2nd hand MacBook suddenly needs an administrator password. Help!
I bought a reconditioned MacBook A1342 online, got it 2 weeks ago and all was well. (I believe I have High Sierra operating system -- OS X 13.) I was connected to my building's WiFi, and added photos from my camera, etc. But this morning I went to use it, and the monitor shows a little photo with "adm" below, and a place to enter a password. Near the bottom of the monitor is an X with Cancel below that.
I have no password. I sent a message to the seller, but just in case seller can't help with the password problem, I'm looking for help elsewhere. When I click on the Cancel "X" the monitor goes black for a few seconds, then I'm back to needing a password -- even though I spend 2 weeks using the laptop without one. I improperly shut down my computer by holding down the power / start button, and then started up again, hoping that might fix the glitch. Didn't help.
I've got a 2002 copy of Mac OS X The Missing Manual, and read that if the administrator forgets the password you can insert a Mac OS X CD, and restart the computer while holding down the letter C key. No disk drive, though I do have ancient OS X Tiger installation disks.
I read other posts about forgotten passwords, and I'm to contact Apple Support on another Mac device, but the only thing I can go online with now is a Kindle. Plus this MacBook isn't registered in my name, so that may be a problem.
Does anyone know what my next move should be? And why am I now needing a password, when I'd been using the MacBook without one?
MacBook