Can’t mount back OS disk partition (no /dev/sda)

I was trying to expand my Azure VM’s OS disk and I’ve unmounted /dev/sda1 which is OS partition in order to do the steps mentioned here: https://docs.microsoft.com/en-us/azure/virtual-machines/linux/expand-disks

But sudo unmount /dev/sda1 didn’t do the job as it said “umount: /: target is busy”. So I’ve unmounted the disk with the sudo umount -l /dev/sda1 command. Since then I can’t even do sudo in terminal and df -H is not working anymore. It throws “df: cannot read table of mounted file systems: No such file or directory”.

When I type sudo, it says “sudo: effective uid is not 0, is /usr/bin/sudo on a file system with the ‘nosuid’ option set or an NFS file system without root privileges?”

How can I fix this issue please help. Thanks in advance.