Can anyone tell me what I did wrong?

[Layspeed]

I think when you changed into manual mode you forgot to adjust your aperture and speed settings. also try checking your metering if its in the middle. Maybe you were just too excited to take the shot and completely forgot to check your camera settings. Hope this helps.

+1 I agree. I've done it a few times before taking pictures of my kids and forgetting to make adjustments after turning the dial.

And I'm sure we'd all like to see what you were planning to take a picture of.