Try using a graphite pencil or wax to lubricate the zipper teeth. You can also try using pliers to gently wiggle the slider or pulling the fabric around the zipper to loosen it.
You can buy a replacement slider of the same size and style as your current one and use pliers to remove the old one and attach the new one. Follow the instructions that come with the replacement slider.
You can use pliers to gently bend the broken tooth back into place. If the tooth is completely missing, you can use a pair of scissors to remove the tooth from the opposite side of the zipper to make it symmetrical. Alternatively, you can replace the entire zipper if the damage is extensive.