knee pain! can 20yr old injury cause pain?!

The popping and cracking you hear is a broken me discus (cartilage) and since you injured it 20 years ago, you now have arthritis in that knee. You should go to your doctor and have him/her check it out to play it safe, but to answer your question, every injury that you had when you were younger and just blew them off, do come back to bite you in the butt as you get older, and since you cracked your me discus, it is a serious injury and you are paying for it now.
Unfortunately, cartilage does not heal since it does not receive a supply of blood, so if it is only your me discus, same day surgery to remove a portion or the entire me discus is necessary so you have full extension and use of your knee. Surgery today is done arthroscopic, so there is no large scars to deal with, just a few small holes are cut into your skin, and you are walking again the next day.

Certified paramedic. and had the entire me discus removed from my knee before the use of arthroscopic surgery

Sure. Surely you've heard of guys talking about old football injuries. It sounds like you have an injury more like a meniscus tear. That may have been your original injury and over the years continued to be aggravated. Think about it, you use your knees all the time and continued working and pressure can bear on an old injury making it worse. The locking up is a bad thing. My dad was a stagehand and he was climbing on a tower when his knee locked up and he couldn't move - had to be rescued. This isn't something that you can ignore - you need to get medical attention.

You could easily injure it simply walking stairs or in a fall, and it could be related to the old injury -- or you could simply have done something new to cause the problem. Best bet would be to say that it has started hurting and causing you problems - not say that it is an old injury, insurance might not cover it if you get my meaning, it would have to be a new complaint/injury, especially if your require more than general treatment.