If your PC is hibernating (rather than in standby), it is powered off. So you must press the power-on button (as if to boot). However, when a PC goes in to Hibernate mode it logs this on the hard disc and writes the memory contents to the hibernate file (e.g. hiberfil.sys). When your PC goes to boot, the hibernate status is detected and the memory information is reloaded from the hibernate file rather than booting from scratch.
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