What happens if you cut your vein?

Aside from being in excruciating pain, you'd lose a bit of blood and would need to be rushed to the hospital. There they'd administer pain relief and attempt to surgically re-attach your tongue. It would likely be unsuccessful.

After recovering, you'd be admitted to a psychiatric hospital to assess your mental health. Your reputation would be a shambles, and your friends and family would be distraught that you could do such a thing to yourself.

You would be incapable of articulate speech for the rest of your life, even if re-attachment was successful. Your social life would have effectively been dealt an irreparable blow, and you would be condemned to a life of cruel taunts, rumours, psychiatric medication and counseling, isolation and speech impediment.