Help, I can't see what I'm typing!
If you've typed into the console and your cursor went to the bottom of it, and now you can't see what you're typing (but it does seem to have an effect) the problem might be that your browser is zoomed in. This confuses the console and it starts trying to display the bottom line outside the visible part of the screen.
You can reset the zoom level to default by hitting Ctrl+0 (Control-zero)
Because the console tends to capture keypresses, you'll need to move the focus outside of the terminal. Click anywhere outside the console but inside the browser window... the URL bar, for example. Then hit Ctrl+0.
Alternatively, you can use your browser's menu to reset it: in Chrome or Firefox, you do this by going to the browser menu (the three horizontal lines at the top right) and set the Zoom (in the menu) to 100%.
One potential way to avoid having to keep changing your zoom settings around is to download an app such as ZoomWheel which can let you customize what zoom level you want for each website.