I have a 75 gallon Bradford White hot water heater. I went to replace the sacraficial anode today, it is also the outet for the hot water. I unscrewed it all the way, but I could not get it to come out. I tried prying it, and tapping with a hammer, but it still wouldn't go. Any suggestions?
It may have become coated with lime to the point where it is now too fat to come out through the hole.
You could, not too forcefully, pump it up and down to try to scrape off the sediment as it comes out, although you do not want to deform the hole that has to accept the screw thread of the new rod.
I'm not sure of any adverse consequences but I would cut it off and let it fall to the bottom of the tank and then install the new rod.
Incidentally, the rod covered with lime is no longer able to perform its sacrificial duty although most likely the tank insides are also covered with lime which according to some books on plumbing offers additional protection against rust.
In the future, inspect the rod more frequently and also scrape it clean, down to the bare metal, if needed before re-inserting it.