Why is all of the text in Rocket League showing as blocks?

This happens when the Rocket League font file becomes corrupted. Follow these steps if Rocket League shows text as blocks █ █ █ :

1. Go to C:\Program Files (x86)\Steam\steamapps\common\rocketleague\TAGame\CookedPCConsole
2. Right-click GfxFonts.upk.
3. Select Delete.
4. Follow the steps here to verify your game files on PC: How do I Verify the Integrity of Game Files on a PC?

If only some text shows up as blocks, but not all, the blocks may be unsupported characters. Unsupported characters are usually text in a different language, like Japanese or Arabic characters.