Things usually go wrong on the way like that ) It was working before and did not in the next moment )
First thing of troubleshooting is to calm down. Then try to check systematically, where the voltage could disappear )
Firstly, check the cables, memory and anything that is attached to the mobo for contact errors, check the integrity of the connectors (corrosion on pins or missing pin, cable loose or so).
Have you got a multimeter? Connect the PC PS to the mobo. Switch it on (just the PS!). Measure the voltage on the purple line of ATX connector. It must be around +5V in standby. Now, measure the voltage on green line. It must be around +5V, too. Then try to start the mobo (with a metal pincer, according to the pinout of those pins in the manual), meanwhile monitoring the green line voltage. It must be near 0V and must remain 0, while the computer is on. If it is not, force it, by connecting the green pole to the black pole by a wire (you can try it from the other -wires - side). It forces the PS to switch on and providing all voltages.
You also can do it with the separate Opus to decide whether the Opus works properly.
Is there any fuse on the mobo? Check them. Are the top of the electric capacitors flat or sphered?
If these are not help, I think it is better to let a technician to look at it.