Sounds like you've go a short somewhere. If by "disconnected just a graphic card" you meant you removed it, make sure it is seated properly. A remove and reinsert may prove helpful. On the other hand, if you simply disconnected the power connector(s), try disconnecting them again and then powering on your system. This will help determine if the short is happening at the power connectors to the GPU. You may also want to try powering up the PC w/o the GPU to see if the shutdown persists even w/o GPU.
Another point to be mindful of is how you cleaned out the dust. PC components can be damaged by static electricity, and often the amount is too little for you to notice, but still deadly for your parts. To rule out any case shorts, pull the motherboard out of the case and put it on cardboard, then reinstall/reconnect: CPU cooler/heatsink, 1 DIMM (start with slot closest to CPU; move outward as necessary, as you continue to troubleshoot), GPU (if you have an integrated GPU, consider using it instead); and SSD/HDD