If your pan falls into disuse and/or suffers an egregious insult (such as dishwashing, heavy deglazing, overnight soaking, or a shipwreck) it will probably develop rust.
If the rust is light, you might get away with scrubbing the pan with a non-abrasive pad, oil, and kosher salt. Once the pan is rust-free, rinse in hot water, and then follow directions for seasoning cast iron.
If your cast iron has developed heavy rust, or the salt and oil treatment isn't up to the task, you'll need to burn off everything and start over.
Luckily, this aspect of cast iron care is easily accomplished, and is kind of fun.
First, you'll need to remove the rust, dirt, old seasoning, or other detritus. The best way to do this is to use your electric oven's "clean" cycle... throw the pan in, and run a full cleaning session.
You can also burn off the seasoning using your propane barbeque grill. Put the pan on the grill on high heat until it is clean... probably an hour or so.
You can also throw your pan in a fire. Build a large campfire and throw the pan into the coals for an hour or so. (Note: on older, thinner pans, this can cause warping. It can also warp the steel wire handle of dutch ovens). If you are not able to build a suitable campfire, a fireplace will work just fine. A charcoal grill also works well (throw the pan on the grill over a hot fire for an hour or so.
Once the pan is totally "naked" wipe off any remaining dirt or ash (be very careful of moisture at this stage, as the naked pan will rust very easily). Now season the cast iron (a few times wouldn't hurt).