Has anything happened while the PC was working, before hissing sound? I mean, was there any power outage in your area during that period? I'm asking you this because if it was, there could be a damage of the motherboard, but still, you have to check that. Have you tried resetting the BIOS? If not, please try do so by pulling out the CMOS battery. You must do that after you have unplugged the power supply unit of the PC.
Open the PC and localize the CMOS battery which lies on the motherboard and it looks like a watch battery. If you have another CMOS battery, you can replace it with that. If you don't, leave your current battery aside and wait for some minutes for the electricity to discharge. Then, plug the CMOS battery back in and the PSU /power supply unit/ too. Try to boot the PC again.
Let me know if that helped.