I have bruised my big toe nail playing squash. How can I treat this and what should I do to protect it when playing squash again?

Accidents happen. if there is blood under toenail and it hurts, a foot doc can drain it for you. If there is no pain, don't worry about it.
Su Ungual Bruise. It all depends on your symptoms. Apply ice if it is painful. This should help with the pain as well as stop the bleeding. If no pain, then leave it alone. The nail may have to be drained to relieve the pressure. Good luck on prevention. These things happen with high foot impact exercising.
You might have. a subungual hematoma which can become quite painful, so you should show it to your doctor. You should wear shoes that can protect your feet and toes from injury.