Have you tried to go Options > Mulitplayer Settings > Chat: Make sure it says "Shown" nest to it. Either that, or it is a bug, the staff are currently fixing a lot of bugs. Hope That Helped!
yup have that checked to shown thx for trying to help STAFF PLEASE FIX i dont get points ofr kills i cant vote
Have you tried shutting off your computer then turning it back on, sometimes this helps, or just reset Minecraft.
Hey, it happened to me too and, it's due to the large amount of players going on everyday. so it doesnt really effect the game, but try relogging.
Try waiting until the server isn't as full or laggy, either that or you have accidentally received a mute or something so you can not read chat, it's happening to a lot of people s i think it may also be a current bug. The staff should be working on it.
Number 1 IT question Mutes don't hide chat from a player, they simply stop the player from chatting in the world chat.