That unit file has been corrupted, it's just containing gibberish now.
You may get lucky by replacing it with the normal content (click for details).
-rw-r--r-- 1 root root 335 Sep 18 13:13 /lib/systemd/system/lighttpd.service
[Unit]
Description=Lighttpd Daemon
After=network-online.target
[Service]
Type=simple
PIDFile=/run/lighttpd.pid
ExecStartPre=/usr/sbin/lighttpd -tt -f /etc/lighttpd/lighttpd.conf
ExecStart=/usr/sbin/lighttpd -D -f /etc/lighttpd/lighttpd.conf
ExecReload=/bin/kill -USR1 $MAINPID
Restart=on-failure
[Install]
WantedBy=multi-user.target
But files don't change their content by themselves, there has to be a reason why that file has been corrupted.
Two common causes would be an insufficient power supply or a defective sd card that's nearing its end of life.
There's also a chance this happened during a sudden power loss or by just pulling the plug on your Pi-hole machine.
You should check your logs for recent occurences of under-voltages:
zgrep -ni “voltage” /var/log/syslog*
If that produces any hits, you should consider a different PSU and/or better power cables for your RPi.
Whatever compromised that file may have done the same to other files as well.
If you have reason to believe so (e.g. if fixing that file doesn't fix Pi-hole for you), you should consider to at least reformat that sd card, or to replace it by a new one.