Secure the Flashtool software and the specific FTF file for the Xperia C C2305 Go to product viewer dialog for this item. .
You can also try a button-combination reset by holding Power + Volume Up until the recovery menu appears, then selecting "Wipe data/factory reset".
The tool will wipe the cache and user data. Once finished, disconnect and reboot the phone; the lock screen will be gone. Official Alternatives (No Flashing Required)