The hashed password is stored in /etc/shadow.
The hashed password is the entry right after the username.
username : hashed password : last change of password : min : max : warn : inactive : expire
So you could try to crack it with tools like hashcat or john the ripper, which will maybe take a loooong long time.
Or you grab the default shadow file from the DietPi image and copy it over onto your SD card, which should restore the password to default? I’m not complete shure if this works, unfortunately I can not test this right now.