This is for people who don't know how to downgrade Minecraft
(Windows 7)
If you don't have a file saying yesterday then click on a file where you had 1.5.1. This could be last week or something.
(Windows 7 and 8 )
http://www.mod-minecraft.org/minecraft-jar-download/Press For MC 1.5.1: Client and it will download instantly
Go to Start search bar
Write %appdata% and press Roaming
Press .minecraft
press Bin
Delete minecraft.jar or minecraft
Put the downloaded file there and your ready to play!
NOTE: make sure minecraft is closed the whole time. If you want to update again then delete all the files in .minecraft and minecraft.exe itself then download everything again from Minecraft.net
Hope this works
!