Unexplained rendering error

*THIS IS NOT A TEXTURE PACK. THIS IS THE DEFAULT MINECRAFT TEXTURE PACK.*

I have no idea what has happened, but it has seriously messed up Minecraft.
Whether it is afflicted with something else (because some other programs on my compute don't seem to be working now), it may firstly be to do with opening a x512 resolution texture pack, one which my computer can't handle from a previous session.
After closing Minecraft, with less then a GB of RAM remaining, I change the options under AppData/Roaming/.minecraft/options.txt to ["Default"] for the texture pack.
Minecraft began to work. And the end result is included in the screenshots. It also became VERY laggy when the text window is open.
No clue how this has happened, but I have never seen something like this before.