There are 2 prominent answers to this:
1) Over carbonation. Can be a problem if the bottles start to explode. Solution when serving: Pour beer into a jug with a lid so that the foam is to the lid level and close lid. Repeat for another jug or two (depending on amount of beer that you want drink). Now pour from jug 1 (with the lid still on). The lid holds the foam back and you can get to your beer! Time will also allow the head to fall back, releasing the beer.
2) Infection. Bad news. Your beer tastes and smells different from what you planned (sour/acidic). Most people do not like sour beers. I would throw it out.
To not have over-carbonation, see @uSlackr's link or use carbonation drops (one per bottle for less than 500ml, 2 for less than 1000ml, etc. If you are using a bottling bucket, sugar or other methods, I can't help.
Infections: Clean and sanitize better. Remember the caps.