Fast start, but quick slow down.

So I stared a batch of pale ale a week ago. Within 12 hours of pitching the yeast, it was going crazy. The airlock was bubbling pretty fast. 24 hours later though, it stopped. I gave the carboy a gentle stir (or shake since the airlock was still on), and it started up again, but its been a really slow fermentation since. All the krausen disappeared after it stopped. Did it possibly just ferment to completion in that one day? My pitching temperature couldn't have been that high. I'm just worried its going to taste of fusel alcohols and phenols.

What do you guys think?

Also my hydrometer cracked so I have no idea of the OG.

That's the only way to really tell is to take hydrometer reading, just cause there is no airlock activity doesn't mean the yeast aren't still doing their thing.

RDWHAHB. An airlock is a pressure release valve not a fermentation indicator. Is this in a bucket? If so its not uncommon for the CO2 to escape from around the lid and not out the airlock. If your worried stick your good ear up to the side of the bucket and listen for a hissing sound. That's the CO2 bubble coming to the surface. Like RUNningonbrew said get a hydrometer and test its gravity.

No. It's a glass carboy. I checked several times to make sure it wasn't leaking. I'm without a hydrometer, so the only thing I can go on is what it looks like and how the airlock is acting. It was just a very sudden change to go from vigorous airlock activity and a thick foamy krausen to no activity and no krausen within a day.