Cider question first time brewer

About 3 1/2 weeks ago I started my first hard cider batch. The batch was fermenting for about 1 1/2 to 2 weeks. I could see bubbling and the airlock was releasing every 3 to 5 seconds but now it is not bubbling or releasing any CO2 from the airlock at all. I transferred to another pail and added a cup of sugar but it did not kick off the fermenting process again. I read that the cider should still ferment for another month or so, but it is not fermenting at all now. I'm not sure where I screwed up. Any help or comments are appreciated.

I used 5 gallons of cider from a local orchard that pasturizes with uv and is preservative free.

Which yeast did you use? Some of them finish pretty quickly. You could be at the alcohol limit for the yeast (depending on your recipe), or could require some additional yeast nutrients, or it could just be a slower fermentation. And again, it could be done. What's your OG and current gravity?

I used Nottingham Ale yeast. I'm not sure if it is any good. I never took an OG. I will have to look at the current gravity when I get home later.

Nottingham seems to be a pretty preferred yeast, particularly for those aiming for a pasturized or cold crashed sweet sparkling cider.
The more experienced brewers will have more information and help. If you didn't add sugar initially, it seems that apple juice/cider tends to fall in the 1.040 range, give or take. Not something you want to use as an accurate gauge though. I think your current gravity reading will probably tell you quite a bit more.