Finding out that you have roof leak is not the kind of thing anyone wishes for but is a problem that certainly needs to be deal with. Making repairs now will prevent more costly repairs down the road.
Many roof leaks occur during the colder months when temperatures hover around the freezing point and melting snow or ice melts and re-freezes causing the ice to back up under roof shingles, usually just above the soffit area. This is commonly referred to as "damming". If this is the case, not much can be done immediately since this area is often covered with a layer of ice and removing the ice could damage the roof. Place buckets or pails under the problem area, preferably in the attic if possible. The leak should stop when the temperature places above the freezing mark or drops below freezing and melting ceases. This problem happens more on older homes that do not have an ice and water membrane underlayment under the roof shingles.
Most of the roofing contractors today use this product when installing a new roof. It is an asphalt and rubber material with an adhesive side that gets applied directly to the roof sheathing. Roofing nails that pass through the membrane form a watertight seal. The membrane is usually installed on the first six to twelve feet of roof depending on the pitch, where most of the ice damming is likely to occur. Some contractors prefer to use it on the entire roof.
Leaks can also occur during heavy wind rain rain events when water is entering where in an area that has not received water in a long period of time. This often happens around roof penetrations such as chimneys and vent pipes. Many times the flashing around these objects has deteriorated over time and has either split or separated from the chimney mortar joint.
The best thing to do if this is the case is to apply a good quality polyurethane caulking on the split if it's in the flashing or if it has come out of the mortar joint, clean the joint out as best as possible with a stiff brush and push the flashing back in and apply the caulk in the mortar joint. This is a temporary measure and in either case you should contact a qualified roofing contractor to make a permanent fix or get quotes from several roofing contractors for a roof replacement.
Your roof protects your investment so taking care of it when problems should be a top priority.