First thing is shut off EL. and water. Open drain valve take a wet vac and put it over Drain valve after you open it and blow air into valve to loosen sediment, open up relief valve to give it air. After it is drained take wire off lower el. and use 1-5/8″ socket and take out element. Take wet vac and tape piece 3/4″ hose about 18″ long and suck out sedimentm after you are done put everything back, make sure to dope threads are use telflon tape. Fill with water and check for leaks and turn on EL. If you cant do all of this call a LIC. Plumber.
Try this, Close the supply valve and open the drain valve. Get a stiff wire (a piece of 12 gauge copper wire works well) and run it into the valve. If done properly, the wire will clear a path through the debris. You may have to do this several times. I have successfully done this numerous times.
I think it’s useful to you “Shut off supply valve and unscrew (remove) the drain valve completely. Have a large bucket or other way of catching the water and junk that will flow. This method is ideal if your heater is located in an area where the resulting flow of water won’t do any damage. One work of caution; some of those cheap plastic valves will break if not careful.” You can get all the information about plumbing here