When putting up Christmas lights inside or out I save myself a lot of grief by doing the following things. First, stock up on bulbs and fuses in advance to save making an extra trip out to buy them. Untangle each string. Plug one at a time in and replace bulbs.
If strand of lights is not working at all, it is probably the fuses. Unplug the lights from the socket. Use a dull bread and butter knife or a coin and slide the cover off the plug, being careful to always slide away from you. Dispose of the dead fuses properly according to the recycling in your area. Slip the fuses in and close the cover. Try again. If both of these helpers still leave you with a dead sting of lights it is better to replace it with a new one than risk a fire. Happy Holidays!