特に起動スクリプトがあるわけではないので、下記コマンドを使って再起動しました。

$ sudo initctl list
rc stop/waiting
tty (/dev/tty3) start/running, process 2914
tty (/dev/tty2) start/running, process 2911
tty (/dev/tty1) start/running, process 2907
tty (/dev/tty6) start/running, process 2921
tty (/dev/tty5) start/running, process 2918
tty (/dev/tty4) start/running, process 2916
update-motd stop/waiting
plymouth-shutdown stop/waiting
control-alt-delete stop/waiting
rcS-emergency stop/waiting
kexec-disable stop/waiting
quit-plymouth stop/waiting
rcS stop/waiting
prefdm stop/waiting
init-system-dbus stop/waiting
print-image-id stop/waiting
elastic-network-interfaces stop/waiting
splash-manager stop/waiting
start-ttys stop/waiting
amazon-ssm-agent start/running, process 17313
rcS-sulogin stop/waiting
serial (ttyS0) start/running, process 2906

$ sudo initctl status serial DEV=ttyS0 serial (ttyS0) start/running, process 2906

$ sudo initctl restart serial DEV=ttyS0 serial (ttyS0) start/running, process 5688