The sad face character ☹︎
☹︎ WHITE FROWNING FACE (U+2639) is the text-symbol twin of ☺︎. Both have been in Unicode since 1993, but only the smiling faces were part of the old IBM PC character set, so ☹︎ has no single-number Alt code. Many phones now show it as a colored frowning emoji.
The emoji 🙁 SLIGHTLY FROWNING FACE was added in 2014 as a milder version. In practice, 🙁 reads as mildly disappointed, while the emoji style of ☹︎ shows a deeper frown.
Choosing the right sad emoji
Sad emoji cover a whole range: 😞 disappointed, 😔 pensive or regretful, 😟 worried, 😢 a single tear, 😭 loudly crying, 🥺 pleading puppy eyes, and 🥲 smiling through tears (added in 2020). 💔 is heartbreak and 😿 is a crying cat.
Tone matters. 😭 is often used for laughing so hard you cry rather than real sadness, and 🥺 can be cute or pleading. If you mean it seriously, add a few words.
Sad text faces and how to type them
The sideways emoticon :-( was proposed in 1982 together with :-), as a way to mark that a message was serious rather than a joke. Japanese-style kaomoji read upright and put the tears in the eyes: in (╥﹏╥) the eyes are box-drawing lines that look like streams of tears, and in ಥ_ಥ they are the Kannada letter tha.
To type ☹︎ on Windows, press Win + . (period) and search for "frowning", or in Microsoft Word type 2639 and press Alt + X. On a Mac, press Control + Command + Space. For more faces, see kaomoji, the happier smiley face page, or the heart symbol page for 💔.