I've just bought a large 7 segment LED display.
I'm having trouble getting it to work. I've tried every possible combination of connections.
Basically i'm not sure which is the gound/power connection over the other ones. There was no information when i got it.
It would be useful if you told us exactly what it is you have bought.
I've tried every possible combination of connections.
Quite clearly you have not or it would actually work when you got the right one. If you indeed have tried every combination then the device is broken.
How is the pinout arranged? If you don't know you can find out. Take a 5V line and connect it to a 470R resistor. Next hold the resistor against one pin and apply a ground to each of the other pins in turn to find out when the segments light. If nothing then repeat this with the resistor connected to ground and the other wire connected to 5v.
If you still get nothing then trash the display it is bust.