Humm, for my own experice, the problem can be in the boot selector from MacMini, if the DVD neither works, this internal hardware can be dammage and it hasn't reparing, but always there is hopem and the mechanical hard disk cans dammage because this hardware has almost 20 years old, in this case you can put other hard disk mechanical or SSD, but you first try to start the MacMini from DVD with MacOS or MorphOS CD/DVD, also if you have an ide adapter, remove the hard disk connect it to this IDE adapter and connect all to one Pc or other computer to know if the hard disk is live.
I write many suggestions but with a windos board you can hold down the C key while the MacMini reboot and with the CD or DVD start the computer, if your computer neither start well, the problem is boot selector hardware from MacMini, that the started with the CD, well the hard disk is dammage.
I hope hel you.