If absolutely nothing happens, as you say, then you must go back to basics. Before worrying About fans etc.
Power Supply, Plug & Fuse, PC's Fuse, Connections to Mains Cable, Connections to the On/Off Switch. I know that this sounds boring, but if your posting is to read as you have written then these are the most likely culprits.
If something does happen when you switch on. Then describe it.
No sorry! Should have made myself clearer. If the fan is working OK, try just connecting the graphics card, keyboard and memory, and starting from there. Obviously you will need the CPU, but do make sure the fan is working OK, or you could fry it.
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