FITTING INTERNAL MODEM WRECKS PC?

My Sister installed an internal modem in her PC all went well until the next day when she switched the PC on there was a pop and burning smell now the PC is dead. Has she blown the power supply or the motherboard?

The PC was a used one from a college sale that she had on 30 days approval if she wanted to keep it. I have examined the PC but cant see any scorch marks or burnt wires. Also why did our RCB (Residual Current Breaker) which is fitted into the mains box not trip?

Why would fitting modem wreck PC?

Any advice would welcome.