Hi Jack,firstly welcome to the forum and leJOS. Secondly please do not post the same question to multiple threads, if people can help they will, asking again and again will not help.
Back to your problem. What is actually displayed on the EV3 screen, does it mention leJOS at all, or just Mindstorms? It sounds like there may be a problem with the way your sd card is formatted, or possibly the sd card itself. How did you create your sd card image, the best way is probably to use the leJOS Windows installer to do this rather than copying the files by hand. If you are still having problems you may need to to reformat your card, I would recommend that you use the following program to do this:https://www.sdcard.org/downloads/formatter_4/
leJOS is still a rather complex system to get working on the EV3, we are trying to make it easier but there is still work to do.