First thing to work out is whether you sign in with a Microsoft account (an email address) or a local account (just a name).
Microsoft account: click I forgot my password on the sign in screen, or reset it from your phone through Microsoft's account recovery, then sign in with the new one. Make sure the PC is online.
Local account: if you set security questions when you created it, a reset password link appears under the box after one wrong attempt. If there's another admin account on the PC, sign in with that and change the password from Computer Management.
If none of those apply there's no official way around it apart from Reset this PC from the recovery menu. Before going that route, if the files matter, take the drive out and copy them off in another machine.
