The main purpose of travel insurance is to cover the costs of emergency medical treatment overseas, and anyone injured in the attacks in Sri Lanka will be able to get help, treatment and make a claim in the usual way.
If you are planning to visit an area where an attack has occurred, you should heed advice from the Foreign and Commonwealth Office here and, where necessary, talk to your travel agent, tour operator, airline or other transport provider. Travel insurance does not generally provide cover for customers changing their minds about a trip unless there is specific FCO advice to avoid the country, so anyone rethinking plans to go to Sri Lanka should speak to their tour operator or holiday firm in the first instance. If you change your travel arrangements, then your travel insurance can usually be transferred to cover the new destination.