I updated my phone with Odin v.3.04 and now it is not working anymore. I took out the USB cable and I removed the battery while the phone was in download mode. When I went to turn my phone on again, it would not turn on. Can someone please help me?
Ok so I'm going to take a small leap here and assume you were trying to root your device. Here is what you need to know:
You can immediately tell that you have a soft-bricked phone if it turns on but doesn't really 'turn on'. You will see some kind of boot loader screen that will prompt you to connect your phone to your computer and start a transfer. Sometimes a soft-brick will look like a hard-brick (see below), in this case you can plug your device into your computer and see if you can connect to it. If you can connect to it and transfer over a new ROM, then your device is soft-bricked.
What is it?
This word is actually miss used all over the internet. When a device is 'bricked', that means that the system can boot up but it can't launch the operating system because it has been erased/corrupted. A soft-bricked device is actually not the worst thing in the world, all you have to do is flash it with a new ROM and you're good to go! Make sure the rom you are trying to flash the device with goes with the EXACT device. 'close' only counts in horseshoes and hand grenades.
You can tell that a device is hard-bricked if you try to turn it on or charge it and you get no response. If you cannot launch the boot loader with the special combination (usually power button along with up volume and menu button) then it is probably a hard-brick. Make sure you have determined that your power source and your battery are functioning properly before you jump to this conclusion.
What is it?
A device that is hard bricked is almost useless. There is almost nothing you can do to get it back unless you have professional experience in hardware hacking. Basically what you need to do is make your own 'Jig' that flashes the device ROM with a new boot loader. Once the boot loader is flashed, then you have your backdoor. You can then boot up the phone and flash it.
I will be completely honest with you, you have 2 options if your device has been hard-bricked:
Try to get a new one through your warranty (which is now void) on your device if it is not expired. If you try to do this and you get caught you will have to pay the full price of the device and your carrier will get very angry with you. If you get away with it, you have a new phone and the phone carriers get to sell your phone as a refurbished phone.
Pay someone to J-Tag it for you. Depending on which device you have, it can cost as low as $40. I DO NOT recommend this method because if they decide that they want to keep your phone (especially if you are shipping to Canada or Asia) there is nothing you can do and you are SOL with no phone and $40 missing out of your pocket.
The internet is full of people with your exact same problem. A quick google search will give you answers to most of your questions.
For Samsung device users specifically
Samsung devices can usually be forced into download mode using a JIG. They are pretty simple and cheap to make. If your device has been soft-bricked this will almost always work. If your device is hard-bricked this method will not work, I've tried.