It depends on your phone. It could be any number of things that would cause it. From not enough free space on the device, to malware or a virus, bad app, etc.
First try restarting (power off and then back on) the phone. You may get the data back. If not, then you can try the following, but there is no guarantees with lost data.
Note: If you have the backup and sync option turned on, then the data may be on your Google account.
There are a few apps/programs out there that 'may' be able to help you recover the data.
A few of the programs you could try would be... DiskDigger, MobiSaver, Android-Recovery and Tenorshare. They are all data recovery apps/programs. There are many others out there, so check around and see which you would...