The hold switch probably got misaligned when you put the iPod back together. You need to open it again, and move the hold switch on the back panel into the same position as the little black switch on the logic board, and put it back together like that. Then, when you move the hold switch, it should work.
Also, before putting it back together, you can turn on the iPod and make sure that the little black switch on the logic board works.
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