I'm playing Gmod TTT and no text is showing up - I can't read my weapons menu, map vote or HUD. This is very strange because I could play just fine yesterday - I played for several hours on the same server as I am now. I also played on a different server a couple of minutes ago and had the same problem. What's going on? This screenshot shows what I mean:
This sounds like a game file was removed or corrupted. Go into Steam. Right click on where it says "Garry's Mod" in the list of games. Click on "Properties". Go to the "Local Files" section. Click on "Verify integrity of game cache". It will show a loading bar for a few seconds to a few minutes, and when it ends your problems should be fixed.
A YouTube video of someone doing this can be found here.