Sometimes when I am taking pictures, or record 60FPS video only, the camera will simply freeze, and if switched off, it won't respond and display what was on the screen right before it freezed. The only way to fix it is to take out the battery... Does anyone know why or how it does this and how to fix it?
Sounds like the SD card.
The camera will first put data into memory / a buffer, then from there, write to the card. If the camera is recording data at a rate that is faster than the card can write, then eventually the buffer will fill. At this point the camera becomes unresponsive. When you turn the camera off, it is still writing out the buffer to the SD card.
Even if your card reports 48 megabytes/second, pay careful attention to whether this is a guaranteed minimum speed. You will probably find it is a peak, or maximum speed, and maybe only for reading from the card, not writing to it. I wrote a summary article on my business website here, if interested.
You wrote that you never really fixed it, but that it hasn't happened lately and that you agreed it may have been a slow card. Did you try alternate cards?