[Galaxy S III Q&A, Help & Troubleshooting] GeorgeGee:
Yesterday I glanced over to my Samsung Galaxy S3 to see the startup logo (as below) just stuck on my screen. The one that says "Samsung Galaxy S3 GT-I9300".

Here's as much useful info about my phone as I can think of:

I have never tried to root my phone, or do any other modding to it

I bought it second hand from a friend, who says they have not done anything of the sort either. I did a fresh install of what I think was Android 4.3 when I got it

I live in the UK and I think the phone is from the UK too, originally tied to a provider but is now unlocked.

I obviously attempted to reboot the phone. I held down the power button and after quite a few seconds the screen went black, then I let go and instantly the same startup logo reappeared. I tried again, without releasing the power button this time, same result.

I tried plugging in the charger, held down the power button, and it then went to a sort-of charging screen. There was a little grey icon on the battery, appearing as though it was not charging.

It wasn't showing the usual charging animation, rather a different one with a greyed battery icon. Holding down the power button on this screen performed the same as with the original logo screen, it just looped.

Here's what I then tried, in order:

removing the battery for 5, 10, 30 minutes. No luck.

leaving it at the startup screen for 30 minutes. No luck

gently tapping the side of the phone, in case the power button was stuck. No luck.

entering RECOVERY MODE and CLEARING CACHE PARTITION. No luck.

entering RECOVERY MODE and WIPE DATA/FACTORY RESET. No luck.

downloaded Kies and connected phone to PC via USB. Kies recognised that the phone was connected but it would stick at the "connecting" screen. I tried multiple USB cables and multiple USB ports, both front and rear of PC. No luck.

I then downloaded Odin and attempted to 'flash' (not good with the terminology, excuse any mistakes I make) my phone using various firmwares I found on the internet. I tried the md5 file both in the PDA and PHONE boxes. I kept downloading "BTU" firmwares(?), read that's what I want somewhere. The file name is "I9300XXUGMK6_I9300OXAGMK6_I9300XXUGMK6_HOME.tar.md5". However I kept encountering various errors, ranging from "there is no PIT partition", to "can't open the serial(com) port". Basically it would always fail.

I tried 5 different versions of Odin - 3.10, 3.9, 3.7, 3.04, 1.85. No luck, same errors.

I then tried uninstalling Kies, ending all processes related to it, and retrying Odin. Still no luck, getting the same serial(com) port error.

I then tried to use a "GT-9300.pit" file that I found online, in the PIT section of Odin. No luck

MY CURRENT SITUATION:

Having tried all of the above, my phone no longer enters recovery mode. When I boot my phone it goes to the following firmware upgrade error screen saying "Firmware upgrade encounteerd an issue. Please select recovery mode in Kies & try again."

It allows me to enter DOWNLOAD MODE, but not RECOVERY MODE. It loops in the same behaviour as previous screens. Even when on charge it shows this screen now.

I have tried my best to include everything I can so I can get more appropriate help. I do not have any experience with phones and I have tried everything I can by myself, hopefully someone here can help! :)