When i click play it won't open :(

[LandoW]

Once the modpack is downloaded and i click play it loads and then, the screen flashes (the window disappears for a split second and reappears) and then nothing happens pls help

[rzcool]

I have a running theory that I have seen in other places that 64-bit java is the result. Another forum member posted the same issue as me, that once they upgraded to 64-bit they started having this issue, and I have no idea how to go back to 32-bit, if you find out please tell me!