So up until today my phone was working absolutely fine, slow after 3+ years, but fine. I've never tried to do anything fancy with it like flashing anything or changing the OS in any way, which a lot of recommended fixes quote as probably causes.
Anyway, I woke up today and my phone was stuck on the start-up screen saying "Samsung Galaxy SIII GT-I9305" and wouldn't move off that screen. If I try to restart it with the charger plugged in it will instead freeze on a static picture of the battery you get when the phone is charging, but will again not progress any further.
I can get the phone into Odin mode and Recovery mode easily enough, but there isn't much to do once I'm in them. I downloaded Kies and it won't connect to my phone in Odin Mode, it knows that a device is connected but it won't actually be able to connect to it. I'm also in the process of downloading firmware and a copy of Odin3, as recommended by this guide: How to recover a bricked Samsung Galaxy S3 - CNET
I'll attach a photo I took of my phone screen whilst it is in recovery mode, there are a lot of errors.
Like I said, I just woke up today to my phone behaving like this, no idea what could have caused all the errors in the screenshot I took at all. I'm baffled.