I imagine the stain is quite dry by now. Here's some options I know work, to be used alone or consecutively:
- Soak in cold water with a handful of salt, then wash as normal. (Optional: soak in bio detergent afterwards before using the washing machine.)
- Cover the stain in a paste of bicarbonate of soda and water. Salt and water is also said to work, though I've not tried it.
- As RedSonja says, the sun is magic. Put the shirt out with lemon juice or a lot of soap.
- Ammonia is great for organic stains. Soak in cold water, 1 tablespoon of ammonia per litre.
- Soak in water and bleach, or put a bit of bleach in the (cold) washing machine. The latter is less effective, but fine for removing faint stains.
- General rule: cold water. Cold washing machine, if there's any stain left. Hot water sets blood. Would-be murderers, take note.
- Don't mix ammonia and bleach. It generates toxic chloramine. You will die.
For future reference, if the blood is relatively recent:
- Stick the stain under a cold tap and rinse until the water is running clear again.
- Wet a bar of soap and rub it hard on the stain. I save the slivers from old bars for this: the sharp edges make it faster.
- When you have a good thick lather, scrub the stain hard between your fists, cloth against cloth. Repeat steps 2 and 3, rinsing occasionally, until you think there's nothing more you can do.
- By this point I've usually worked even very heavy bloodstains down to a faint yellow mark. Use one of the other methods to finish the job. (If you're going to wait overnight, put it in a basin of water so it won't dry out.)