So much depends on the type of spring as far as safety is concerned. If the springs run along the side of the door when it is open you can easily replace these and they are available at any hardware store. Take the old spring with you and remember the new one will probably be a little shorter due to stretching. If the spring is directly above the door you need to call a professional. They are dangerous. All springs should be replaced not just the broken one.

The springs are under the least amount of tension when the door is open. They can break but with less risk.