Here's how I'd start off. First, disconnect the monitor and hook up the amp per normal and try it. I assume you've already done this, and that's when you got the horrible cranking noise. Next, I'd measure the resistance of the speaker, just to make sure it's all right (no strong reason to think otherwise at this point, but it's an easy check). Try a known good amp on the speaker. If it works, then you've proven the speaker and the cord. Try the 200S on a known good speaker (if the 200S cabinet tested good, then you're already there, as you tested it above). If you've reached this point, then you've proved that the problem is in the 200S head.
The head powers on, so you know it's not the fuse. loudthud mentioned flashes of light in the big tubes. Take that a step further: do they look normal? I had one go bad a few months ago. It had white ash inside. The other didn't. The two power tubes didn't look the same. I don't think one bad tube is your problem, though. Both, maybe.
Do you have spare tubes? If so, replace the tubes. If the amp works, then replace the original tubes one at a time. That is, turn the amp off, let the tubes cool, take one out and put the original back in. Turn the amp on, then test it. Tedious and time consuming, but effective. When the amp stops working, then the last tube you put in is bad. Replace it.
Everything I've described so far, anyone can do. If replacing the tubes doesn't solve the problem, then it's deeper, and more specialized knowledge is required, specifically an amp tech. I assume that you are not an amp tech, so, at this point, I'd suggest you take the amp to someone who is.
