Whenever I try to launch Minecraft (any version) it stalls and won't open.

When I launch Minecraft, I expect it to move past the Mojang logo and go straight to the main menu screen, but instead it stalls and freezes, and I am forced to close it.

I am not sure why this is happening. I have used mods, but it hasn't affected my game. I removed them, changed the version, but it doesn't help. When I check for crash reports, there are none.