Walinen :
My pc doesn't have pci express cable, i red on internet that the slot has enough power so i don't need it even though my card has slot for it and yes its overclocked. So is this a lost call ?
Nope. That's your problem though. If your GPU card has a provision for a 6pin or 8pin PEG connector for power, it MUST be plugged in.
When I looked up the specs on your computer on the HP site, it listed your PSU as a typical 300w unit (read: cheap). Now, since this is only a 750ti and doesn't draw gobs of power, you should be able to use a molex -> 6pin adapter without issues to supply the power for the card. If this were a higher power card like a GTX 950/960 or higher I would not recommend it, but odds are that this card draws barely over the 75w PCIe limit and as such it had to have the 6pin connector and associated circuitry.
Get a power adapter, connect it to one of the molex connectors (hopefully your PSU has one of THOSE!) and your graphics card and you should be all set.