It is possible that the mirror you use to fetch packages is down.
As for the GRUB password, it is written in /etc/grub.d/01_PASSWORD, which you can read from any Live system. Not much of a security... and the GRUB developers agree that "requiring authentication at the boot loader level would only serve to make it difficult to recover broken systems" without adding any additional security in most cases: https://www.gnu.org/software/grub/manual/grub.html#Security
Assuming you can boot an older kernel from GRUB's "advanced options", you can then get rid of GRUB's password by commenting all lines of /etc/grub.d/01_PASSWORD (i.e., using with administrative privileges a text editor to have all lines begin with "#") and then executing 'sudo update-grub'.
Tell us whether an older kernel brings you a usable system. However, if you are certain that the kernel was not updated between the last non-problematic boot and the first problematic, then it must not be the problem. Maybe a hardware problem and you could test your RAM with memtest86+ and your partitions with SMART auto-tests.
