Help. Whats wrong?

For one your soil is too wet/heavy..there doesn't appear to be any aeration. Why isn't there any perlite in your soil? Also if you bought that soil at a store, it might have a lot of added fertilizers, which isn't good for a cannabis seedling. 
 
My advice to save that plant is don't overwater it..the soil looks almost like mud so I suspect you might be watering a bit too much. How big is that container? 

 

IFourTwenty is right, looks like its being over watered. What light are you using? The yellowing leaves could be cause its not getting enough light. It could also be a nutrient deficiency, but im not sure about that cause its only a little seedling, but overall that plant does not look healthy. When its a seedling like that you should only be watering with a spray bottle, the top layers of the soil should be moist, and once in a while spray the leaves. What soil is that? it almost looks like soil you found outside.  

 

 
In the future remember seedlings need aeration for their roots..that's why it's not good for your soil to always be wet, no oxygen can get to your roots. 
Just to give you an idea on how often to water..these plants and seedlings here in this tent get watered once a week or every 5-6 days. If you are watering that seedling anymore than that, you are overwatering. Cannabis isn't native to swamps..remember where cannabis comes from, dry mountain regions.
 

 

they are fried.
 
toast.
 
Miracle Blow soil is the shit.  If you absolutely have to use it, then you need to flush the living shit out of it, like 20 times. Seriously. 
 
Just go buy a bag of compost and the same amount of perlite.  mix 'em both together, plant your seed and relax.  Water will be your next problem.... just say screw it and get the Reverse Osmosis now or at the very least a Zero Water device.
 
enjoy the ride.

 

 Yeah but we all learn this shit at some point most often through experience... I had a whole room of 2 week old seedlings die when I was younger and now I know it was because the soil was way too potent for the little babies.
 
You are probably better off just scrapping that one and starting another. if you want you can steal my set up I use for seedlings... you can literally just let them grow for 2 weeks without ever having to touch them. I use a snapware container that is about 2' deep and 5" x 8. Taje straight perlite and fill up the bottom inch of the container. Then take some 2" coco pots filled with peat, perlite, and EWC (3-3-2 ratio) and nest them into the perlite about a half inch. Cover the top with hydroton and then saturate the perlite with dechlorinated water. Capillary action will cause the water to wick up into the soil in the coco pots as needed and will maintain the perfect moisture as long as the perlite is kept moist. This is great because you don't have to water directly into the seedlings container which can damage the seedling and roots. Here are some seeds that broke soil on Friday afternoon of last week. The pic is from yesterday so just under 4 days after sprouting and they are already starting to form second set of true leaves. The second pic shows a couple Auto Bubblegums that sprouted 10 days ago (the 2 in the middle) and they are already starting in the 4th set of true leaves as of this morning (pic taken yesterday)

 

Make sure there's drainage holes in the bottom of your pot. This one time, some asshole sold me a buncha blueberry bushes and I will have you know I lost most of them because he'd neglected to transplant them into pots that had drainage holes. It'll look just like over-watering would, but you will have literally suffocated your plants.