Have you installed any tweaks before this started happening? You may need to uninstall those tweeks... but in order to do that you need to be able to access cydia. 1. If your memory is low, try going on iTunes Store on your iPhone and search for the movie (with a bigger file size than what you have left on storage. ex: Lord of the Rings), click rent and soon you'll get the message saying you don't have enough storage and offers you to go to settings. Click settings and magically you will get some storage back (it erases some useless data from different apps to try to download the movie). Repeat the process 4-5 times and before you know it you recovered 1 to almost 2 Gb depending on your iPhone memory size. 2. Delete the Pangu/PP Jailbreak app and reboot your device. Once it turns back on, Install Pangu/PP Jailbreak app using Cydia Impactor and accepting the Developers Cert. then reboot you device. After the reboot try to use the Pangu/PP app to Jailbreak your device. Once Jailbroken, use cydia to remove any of the tweaks you recently installed that may have cause this problem.
3. If this doesn't work for you, try to repeat the process 2-3 times. (It doesn't always work on the first try.)