I included emoji in my password and now I can't log in to my Account on Yosemite
If you have "Other Input Sources" available at the top right of your login screen, select the one called Unicode Hex Input.  This can be used to input emoji (or any other character) into the password field, as long as you know the Unicode Hex number of the character.  This number can be found in the Character Viewer or on the internet.

Some items you find in the  "emoji" category have Unicode hex numbers with just 4 characters, such as Airplane U+2708 ✈.  With the Unicode Hex Input keyboard, you input this by holding down the Option key while you type 2708.  

Other emoji have Unicode hex numbers with 5 characters, such as Grinning Face U+1F600 .  For these you need to find the two corresponding UTF-16 Hex codes (sometimes called "surrogates") by consulting Character Viewer or using an internet source like fileformat.info.  For 1F600 these are D83D and DE00.  You can input 1F600 by holding down the option key while typing D83DDE00.  You may see two dots in the field, but it is still just one character.