the only 3 ways to get it back are
1. turn it off
2.go to the move tutor
3. use a TM
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Any Pokemon that you teach Strength.
First, you have to get the HM strength, then teach it to and a Pokemon and you can move boulders.
Actually to teach this move to Gengar you need to find the move tutor that will teach the move to any of your Pokemon this move tutor is in Viridian city you need the HM Cut to get to him though. Also a move tutor in Pokemon xd gale of darkness will teach Dream Eater to your Pokemon.
It depend what game you have. In Pokemon Ruby you can buy the move in Lilicove City(same with Emerald and Sapphire ) In Diamond you buy the move in Veilstone City.(same with Pearl and Platinum) After you get the move just press A and then select use. Then choose which Pokemon you want to learn. Or want till gets to a certain lvl
you teach a Pokemon that move or use a TM