You will be able to revive 90% of the fallen troops using gold, if they died when attacking another player or a monster. If the troops instead died when defending a location from an enemy, you will be able to revive 75% of the fallen troops using silver. As an alternative to gold and silver, you can also use Sacred potions produced by your Temple to revive any fallen troops.

The cost of reviving troops, as well as the production of Sacred potions, depends on the level of your Temple. Various active tournaments may affect either the currency used to revive troops, or the cost itself. Ruthless Slaughter allows you to revive all troops that died in attack for silver instead of gold, while the Annihilation Fever decreases the cost of reviving troops that died when attacking other players.
Note that if only one troops dies in attack or defense, you will not be able to revive it.