If your Hipstamatic viewfinder is black/blank/closed or the app is crashing when trying to take pictures, then it's likely that you're running into the Privacy setting added by Apple in iOS 8.
That setting is easily update though!
Just open the Settings app on your device and look for the following location:
Settings → Privacy → Camera
Just make sure the switch next to Hipstamatic is flipped to “ON”
Then do a safe restart of the app (no need to reinstall) and see if things are working better.
If the instructions in this thread haven't helped, please send us an email to: firstname.lastname@example.org. Provide as much details as you can about the specific situation you're having on your phone/device.