I cut my thumb with a little knife but it was a little cut but I cut it on my veins I think, I can't see things clearly and hear, what's happened?

Should be looked at. If a person cuts himself but has poor vision, he needs to get somebody else to look at it to see if the cut needs stitches. Usually, if the cut is doesn't go through the first layer of skin (about the thickness of a coin), it should heal up by itself. A little deeper than that means it may need to be stitched or glued back together.