A reason? Disk failure. Having only a single file can be tricky. All it takes is a simple error in the file table and what that file contains is now unknown and potentially unrecoverable.
You can run chkdsk to check your drive for errors. Right click the drive > Properties > Tools.
You can run GW2 with the command “Gw2.exe -repair” to try and repair it.
In the future, you may want to create a backup of Gw2.dat, so if it happens again, you can simply copy it over.