Step back and take a look at the whole roof. Look for missing shingles, large areas of discoloration or sunken areas in the middle of the roof. Discoloration could signal an algae problem and a sagging, sunken roof could mean rotten plywood below.
Clean all the debris from the roof, being careful not to damage the shingles. Debris holds water, which can deteriorate the roof. Sweep away the debris, but be gentle on a roof with asphalt shingles. The granules on asphalt shingles come off very easily and if enough get knocked off the shingles will be damaged.
Inspect the keyways for deterioration.
If there are loose shingles, use roofing cement to re-secure the shingle to the roof. Chances are if one loose shingle is found there will be more so make sure there is enough roofing cement.
Take a closer look at the shingles to make sure they are in generally good condition. If warped or curling shingles are found it is time to think about replacing the roof.