I am locked out of my laptop after uninstalling Windows 10, how can I get back in?

[Windows Central Question]

locked out of my laptop after uninstalling windows 10

hi can anyone help please?
I bought a new hp laptop a few days ago , it came with windows 8.1, I did all the usual to set it up ie administrators password, windows 10 update appeared so I downloaded it but it was making my computer run very slow things not working right so I uninstalled windows 20 and went back to the original windows 8.1 but since then I cannot log into my laptop even after resetting my password with Microsoft it still wont allow me to sign in...........help someone please