gibson 12 string electric guitar

Problem with restringing guitar, please help?
So I’m restringing my string-thru, Gibson headstock-style guitar, and I’ve suddenly encountered a problem that makes my electric guitar unplayable.
The strings all end up resting on the first fret. As you can guess, this means that half the frets under the 12th fret are completely unplayable and the string buzz is overwhelming.
I’m not the most experienced guitar player so I have no idea what I’m doing wrong. If anyone could give me tips to follow when restringing, or maybe they understand my problem, that would be highly appreciated as I’m about to give up on it.
There’s not much that can go drastically wrong mechanically when restringing unless you accidentally “adjusted” something! Assuming you are using similar gauge strings, have checked the bridge height, saddles and nut. Are you positive it’s tuned to standard? The problem you describe could easily be caused by switching to a lighter gauge of strings and not being tuned to up to standard – this could not provide enough pressure on the neck to pull it straight resulting in a little bow/ buzz.
Next I would ask if you have an adjustable bridge try raising it slightly and see if it improves. But it really sounds like when you unstrung it your neck may have bowed a little, which is normal, and the trick is gettting your new strings to straighten it out. This issue occurs more often in lower quality guitars . You didn’t mention the brand, string-thru (the back?) like a Fender but Gibson style head/pegs. Good luck!
Original 12-String/Electric Guitar Solo