s4 galaxy phone update gone bad my phone was updating but was disconnected.Now it is only showing " software update failed" with a phone symbol and a command sign with a computer next. whats has gone wrong?
Since updating process didin't complete as expected, Then it's normal to see that sign, In order to recover the phone to it's original state, You'll have to flash it with it's original Firmware using the official tools from Samsung called "Odin", You need two thing (Odin and Firmware), For the Firmware you can always download any of Samsung Firmwares from Sammobile -they need you to be a registered user but it's free-, And download the latest version of the Firmware and it's 5.0.1, Now for Odin you can download the latest one from Odin_All_Versions, Once you have all the files needed you can start the process by following this guide, 


Extract the Firmware and Odin to the same folder
Power off the Phone
Boot the phone into Download Mode by pressing (Vol-, Home and Power button)
Connect the phone to the PC using USB cable
Run Odin as Admin
Click on (AP) from Odin then choose the Firmware file
Click (START) and leave it do it's things 
After process done you'll see a green "PASS" sign
Now your phone will Reboot one time to "Recovery" and the second one to "System"