What if I have a cessation on Gengar( poke power - once knocked out 3 damage counters to any pokemon) will cessation's effect dissapear once Gengar is knocked out by a pokemon's attack? Therefore I can use its poke-power?
no, u can't use it's poke-power because it's already stated that "As long Cessation Crystal is attached to an Active Pokemon, each player's pokemon CAN'T ANY USE POKE-POWER or POKE-BODY"...
Gengar can NOT use Shadow Curse in this situation. The reason for this is because Cessation Crystal was in effect at the time Gengar was Knocked Out, which is the point in time Shadow Curse's effect would come into play.