Ambulance.
With medical emergencies dial 155.
Health services in the Czech Republic are either public or private. Emergency treatment and first aid are free for everyone visiting Prague.
EU Citizens may get rather cheap medical service if under reciprocal health care treaties. Some other countries may have similar international agreements, so you might want to check with your national authority to find out.
In the case of medical treatment your passport and a valid insurance is required. Without insurance you will need cash or your credit card.