PSU of bad quality and adapter 6->8 pin can be an issue but...
Did you check the card elsewhere? Was it working fine with another PC?
If yes, then maybe You can lend for a while some other, better PSU to test it with Your mobo and GPU?
Maybe the card itself is broken?
Also, do You have some other screens attached or only one? If yes, check the second screen first, maybe it displays data there (TV for example)
Before buying new psu, You should exclude other possibilities:
- damaged card
- damaged PCI-e slot
- card not properly put in PCi-e slot
- all cables plugged where they should be
etc etc
EDIT: If I were You, i would take my rig to some computer service or some friend and ask them for 2 minutes of their time - plug 8 pin connectors from other psu to Your graphics to check if it works. They should do that much for free.