Sounds like a dodgy "hybernate" file.
When (newer) PCs boot they look to see if they are recovering or cold-booting - if there's a hybernate file there they will try and load that and return to the state they were in prior to hybernation... Clear as mud?
If the hybernate file is corrupt, the PC hangs until it is forced to cold-boot... eg by pressing the hard-reset button.
Next time powered-on, the dodgy hybernate file still exists, so lets keep playing this game forever...
When you say no hard drive activity, do you mean NONE, or a tiny bit then nothing more?
HTH