I had this happen, and it took me around 2 years to get the crown put on. Because of the root canal, and the fact that I had shattered half my tooth (hence needing the root canal), there was nothing to protect the internal part of the tooth, and it rotted and decayed from inside. By the time I got it fixed, they had to remove most of the tooth down to the gumline, and instead of having a partial crown attached to mostly real tooth, I now have a full crown attached to a partly real root.
Get it fixed as soon as possible. The longer you wait, the more of your tooth you lose. I'm not sure, but there may also be some risk of gum infection if the decay bores down below the gumline where you can't clean it. See how much it costs to get the tooth cleaned and a new temp filling put on it, then be very careful with the tooth. I believe my crown cost somewhere in the neighborhood of $2000, though it was paid for by insurance so I may not have heard right. If a temp cap is like $200, you may be able to afford that while you save up or get dental insurance.