Troubleshooting old Dell T3400

I was given an old Dell T3400 (label on motherboard says cn-0tp412-7821-854-j1ez) which worked fine for a year until one morning (it was left on overnight), the CPU fan seemed to be at full speed and there was no image on the monitor. The PC speaker does not beep at all (I don't recall whether it ever did but it does exist and is connected). It came with three memory dimms which I've tried individually in different slots and I've tried two different graphics cards. I also tried a different PSU from another Dell system although the original one gives okay 5 V and 12 V (I didn't check other voltages). I cleared the CMOS by unplugging and removing the battery for over five minutes.

The light on the front is orange when the machine is powered on. I'm not sure if it used to be green? There is also an orange LED at the back fo the motherboard, below the I/O and above the first PCI slot. I don't have another proprietary Dell internal PC speaker to try and I don't think it would work with a standard internal PC speaker, right? The dell one has four or five wires.