If a gutter sags too much, it looks unsightly, and if the sag is in the wrong direction, water collects, giving mosquitoes a breeding ground and creating leaks. It's standard to pitch gutters 1/16 inch per foot (2 mm per 30 cm) toward the nearest downspout, but even a level gutter will drain. Assuming that the spikes, brackets or hangers are installed correctly, most commonly a gutter sags due to ice and snow damage or because a ladder has bent or dislodged its supports.
Straighten a bent spike by pushing up on the gutter at that location with a pry bar (you'll likely need a ladder to reach it); or remove a nailed gutter spike for replacement by locking the head of the spike firmly in a pair of locking pliers, then tapping the side of the jaws with a hammer to pull the spike out.
Replace a spike that has fallen out or been removed with a threaded gutter spike. For a tighter fit, press a few wood slivers coated with exterior adhesive or epoxy into the nail hole first.
Remove the gutter by unclipping the brackets and disconnecting it from the downspout(s). This is definitely a job for two or more people.
Replace the damaged bracket. Fill the old screw holes with caulk or exterior putty and let dry. Install the new bracket at approximately the same height and position as the old one. Repeat as needed for other brackets and tighten any loose screws before reinstalling the gutter.
If the strap is bent down, press down on it about 1 inch (2.5 cm) up the roof from the edge of the shingle it lies under, or from the surface to which it's nailed. Then pull up just below the point of the bend.
If the bracket has pulled out of the roof or needs replacing, and the roofing is asphalt shingles, disconnect the bracket from the gutter and carefully pry up the shingle to remove the nails (or screws), or drive new fasteners through the strap into the roof.
Remove the mounting screw to replace a damaged bracket.
Assuming that your building is reasonably level, measure up from the bottom of a fascia board to level or pitch a gutter. Leaning a ladder against an aluminum gutter may ruin it. Use a standoff ladder attachment.
The distance from the foot of the ladder to the house wall should be about one-quarter the distance from the ground to where the ladder leans against the house.