iMac (Intel) shuts down unexpectedly, a semi-loud 'boom!' is heard.

Unplug it immediately and don't plug it in again until it's been serviced.

It sounds like a capacitor blew.

As Linc suggested it is time to take it in for service.

Allan

Even the small caps in today's computers can make a nice "pow" noise and let some seared electrolyte out to wrinkle your nose.

The good news is that usually in this case, the offending part is easy to find for any technician worth his/her salt.

It's not normal use that causes caps to fail. It's either age or a manufacturing defect, usually an impurity in the electrolyte that allows a resistive bridge to build up until the ohmic heating causes a short, then kerpow.

Get it looked at and corrected by a service provider.

It's not a car! You don't have to take it in for periodic service!

In this case you've had a physical component fail (a capacitor). That can only be fixed by physical access to the innards of your computer.

Normal software maintenance is carried out by the OS itself without bothering you. Under normal circumstances it should never need to leave your desk!