[~]# cat /etc/systemd/system/checkhung.timer
[Unit]
Description=Check if file not modified in a while and restart service

[Timer]
OnActiveSec=60min
OnUnitActiveSec=60min

[~]# cat /etc/systemd/system/checkhung.service
[Unit]
Description=Check if file not modified in a while and restart service

[Service]
Type=oneshot
ExecStart=/usr/local/bin/checker.sh

[~]# cat /usr/local/bin/checker.sh
#!/bin/bash

if [[ $(find /path/to/the/file.txt -mmin +60) ]]
then
    /usr/bin/systemctl restart my-service.service
fi