Your Nokia Lumia 635's camera is not working. Here are the three most common issues for you to look at:
Low Battery
Your phone will not let you access the camera if it has a low battery. Make sure the phone is at least over 20% charged.
Missing Camera Icon
It is possible that the camera icon is unpinned from the task menu. You have to scroll to the bottom of the task menu and click on the right arrow. Then scroll down until you reach the camera icon, hold it, and then click on 'pin to start'. Your camera is now available on the task menu.
Broken Camera
After discarding the previous problems, it is possible that the Nokia Lumia 635's camera could be broken. You would need to purchase a new camera, and if you want to replace it by yourself click here.