I have an iPhone 8g 3G 3.1.2 I think the firmware was 4.8 something. I downloaded blackra1n & it installed ok then I clicked on install Cydia it seemed to be ok then my screen went black. When I push the home button or power button I get the silver apple icon but that's all. after a few seconds it goes black again. I tried holding both down & nothing happens just goes to black screen again.
What happened ? How do I fix this ?