Safest Steps to turn on water heater that has been off for 5 years
You might want to open the water line only slightly and let it fill slowly so it won’t have a shock of pressure. I would open the lower drain and get rid if any gunk that may have developed, and check that your anode rod is still there and not corroded away (if it was drained the anode would not have continued to corrode). 

Once it is flushed and fully pressurized, open the valve all the way, then close 1/2 turn to preserve the packing.