After hooking up the PA for practice, I heard a loud "pop," then followed by a burn smell, from my Mackie SRM 450 speaker, which I use as a monitor. No sound comes through but static and crackling. Is the speaker blown? Is this a problem that can be fixed? And, should this speaker not be used as a monitor, could that be why it blew?? Thanks!

The pop itself isn't necessarily a bad thing, but a burn smell and the fact that it's only outputting a crackle likely means that the built-in amplifier circuitry has been overloaded. This can happen a few ways:
1) Faulty power, either a bad cord or a bad outlet
2) Plugging the output of a power amplifier into this speaker - because it has a built in amplifier, this speaker only wants a non-powered line input
3) If the Thru output is (accidentally) plugged into anything that provides phantom power, that could overload the circuitry.
4) A short due to liquid or moisture contact - these cabinets are most certainly not waterproof
In any case, the amplifier part of the circuitry IS replaceable. If you're not comfortable buying one and replacing it yourself (I'd recommend eBay), take it to any shop that sells similar items. If you're extremely lucky, it was just a fuse that needs to be replaced, but chances are you'll need to replace the whole amp.