Maybe it will work and maybe not. A lot depends on the capacity of the 12 volt output.
And maybe it will last a while and maybe not.
And maybe when it fails, it will fail gracefully and electronically shut itself down. Or maybe not.
Maybe it will fail catastrophically, explode or burst into flames. And destroy part of the computer it is plugged into. Or maybe not.
Say. How much is the rest of the computer worth?