How to put my pc on the lan?

we have a home lan with win10 machines on it.
i have a laptop, lenovo thinkpad, on the same lan: uses the same router.
in windows explorer the laptop can see the other computers and their shares.
from another computer I can see the laptop in windows explorer but I don't see its shares.
the win10 system shows me a login screen for the laptop.
i input the username and password and it tells me they're wrong. will not log me in.
I use the username I think I have and that 'whoami' on cmd on the laptop says I am.
I use the login - a four digit number - that i always use on the laptop.
how to troubleshoot this issue?
p.s. the laptop can see the win10 pc and can log in to it.
it's just I can't do it from the win10 end.