In addition to the reboot, as Nikkisharif mentioned, check your phone's memory. How many gigabytes of memory do you have and how many have you used? (Check under storage, which is found under general settings. ) If you have very little memory left apps will act up like this too. You might need to delete some apps you don't use or back up your photos(and then delete from your phone) to a cloud service like iCloud, Dropbox, Googles cloud service, or others, in order to free up memory and give your apps room to work.