So there are a couple of possibilities here, but first, you need to take a gravity reading. If your gravity is either 1.000 or less or your ABV is at/outside of your yeast's tolerance, then it's done, not stalled.
Now, if your gravity does not indicate that your fermentation is complete, then the amount of sulfites you added is incredibly important. If you added 5 ml of sulfite powder (~1 tsp), then that was way too much sodium/potassium metabisulfite, and could potentially kill your yeast. Keep in mind that one Campden tablet is somewhere between 1/16 and 1/8 tsp of sulfite, so dropping 1 tsp into 1 gallon would be like using a steamroller to iron clothes. If you just dissolved 1 tablet's worth into 5 ml of water, though, then that would have been fine.
Okay, so if we assume that the cider is still grossly over-sulfited (you should detect a slightly sulfury smell and flavor), then before you try pitching additional yeast, you need to get that sulfite out of that cider. There are a couple of ways I know of to do this:
Splash racking - The gasses released by sulfites dissipate readily when shaken around. Splash racking one or two times can help to force these out. Essentially, just rack from the top instead of the bottom and let your cider run down the inside of your fermenter.
Copper - For chemistry-related reasons that I'm not in any way qualified to explain, dropping copper (please don't use pennies, they're gross) into a solution full of sulfites for a week or two will cause some reactions with the sulfur compounds that effectively pulls them out of the solution. I've also heard of people racking through a 100% copper scrubbing pad, effectively combining splash racking with copper.
After splash racking and/or racking off the copper, let the cider sit for 24 hours under an airlock. After that, you should be safe to pitch more yeast. Regardless of the yeast you originally used, I recommend pitching Lalvin EC-1118, as it is frequently used to restart stalled fermentations. Lalvin K1-V1116 and Uvaferm 43 can also be used.