A 50 gal. Electric wtr htr has two heating elements and one thermostat,there is usually a red reset button on the thermostat,make sure pwr to thermosat then to elements (if elements had pwr with no wtr in tank,they are toast,try reading continuity thru elements with pwr off,and wires disconnected (this will tell you if elements are good,if so check pwr on line (coming in from pwr source)and if you have pwr ,check the load side (pwr going to elements)if all is good with thermostat pwr in and out and both elements have pwr and are reading thru (continuinty)make sure temp dial is not at max (could have high temp shutout)remember 50 gal is gonna be 220v (black wire-line 1 hot wire,red wire-line 2 hot wire,white wire is usually neutral,but if there is no red wire,it could be your 2nd hot wire and green is ground wire,make sure it has a good ground.
Hope this helps,and good luck......