about 3 weeks old and the main stem cracked when plant was knocked over

should i put it out of its misery or should i nurse it back to health

You can splint it or tape it or tie it up. Pics?

Pics will help, but here is a simple guideline: if it is bent, it will mend; if it is broken, it won't make it. Bending is no different than LST, and actually makes the plant stronger. But if the stalk tissue has torn it cannot repair itself. Are there any branches below the bed/break?

no it was fairly low but it doesnt seem to be all the way through.I will try and get a pic of it tomorrow when i go to the spot i might tape it and see what happens it was a free plant anyway.but its white widow

Doesn't sound like you will lose it- split it and load a pic. Hop it comes throuh, white widow is some nice smoke.