Murky beer, no bubbles in airlock?

From your discription it sounds like it never started fermenting or it stopped. Were there signs of fermentation before? In the first two days especially? Did you take a gravity measurement? Measure again and see if your measurement is close.

Heavy weight beers and lagers sometimes need more yeast and I keep my brews a week or more after signs of fermentation stops. You may have nothing to worry about. Check your gravity, check your seal at the top.

If there weren't bubbles, or signs of fermentation then maybe it didn't start or it stopped early which it can for various reasons. I would recommened making a starter and re-pitching. Starters take a day or more to make... Boil a mixure of 1 quart water, 3.2 ounces DME, 1 pinch yeast nutrient and 1 pellet hops(optional for the hops). Boil for 15 minutes and pour into a sanitised container and cool in the fridge. Add your yeast and shake to add some air. Shake four times over the next day or buy one of those yeast stirring plates (I hear they can get more than 4 times your yeast but I haven't tried one myself...) and then use for pitching your yeast again. The problem though is that starters run about a quart in volume so it maybe a problem if you don't have room in your fermentation vessel. Sometimes it'll need a blow off valve.

Other solutions include aeration, gently swirling the jar, pitching a new dose of the same yeast, adding yeast nutrient, warming the jar a little if you think if got too cold and adding a crushed Beano tablet. I've never tried the last but in a few books it says it'll break down complex carb into sugars which lets the yeast eat again and start fermentation again. They make a note that it may throw your final gravity really low.

I can think of two possible scenarios:

1. The beer has finished fermenting, and you just didn't notice it. If it's relatively warm (say, around 70^o F.), a regular strength beer can finishing fermenting in as little a 2 or 3 days. Once the yeast is done consuming sugars, you won't see much activity in the air lock. In general, air-lock activity is a poor indicator of fermentation. If your fermentation vessel has a leaky lid, for example, you might not see any bubbles in the airlock. If the beer is, in fact, done fermenting you should be able to see evidence of this. There would be a ring of crusty gunk above the surface of the beer. This is the remains of the krausen that formed during fermentation.

2. The beer never started fermenting. It would be good to know what sort of yeast you used, and how you added it. If the yeast was very old, it might fail to ferment the beer. If the wort was too warm when you added the yeast, that could kill it.

You could take a sample of wort and measure its specific gravity to see if fermentation has started. After 8 days, I would expect the gravity to be near or at the target final gravity. That being said, by taking a sample you run a small risk of contaminating the wort, but it's the only way to be sure.

If you think that the beer has not started fermenting, I would suggest that you immediately pitch another packet of yeast. Check the manufacture date of the yeast to be sure it's not past its prime, and follow the instructions on the packet. Typically, this involves re-hydrating in warm, but not hot, water before pitching.