System-breaking changes happen when mucking around as root on a linux system.
Fixing a system that won't boot into a serviceable state is often a simple operation. Provided you have an idea of the operation you performed before rebooting that broke the installation, all it takes is an alternative bootable drive, say a USB with an OS installer. Many default installation .isos don't come with a full-enough featured enough shell to perform fixes but many others do ("live" installers). This guide details how to use a distro's installer shell to fix a broken installation of an arbitrary distribution, for example using a NixOS installation image to fix a broken Debian installation.
Show block devices to find the identifiers (e.g. /dev/sda1) of the broken system's partitions:
# lsblk