I'd suggest a 30 second or more battery pull (do NOT turn the device off first). Failing that, a reload of the latest official OS might be the best solution...
I had this phone for a while before i got my Z10, my advice is also what the two other people mentioned does help however...
This problem will usually occur quite regularly if your application storage memory is very low. The phone will become very irresponsive. Try deleting some big apps that you dont use anymore and get a better memory card too so you dont have to store so many files on your device memory. But the main thing is deleting some apps if your are low on application memory.. its the same memory that the OS runs on.