It's quite difficult to trouble shoot piping problems over the internet. It could be anything -- something that got into the skimmer, like a plastic toy, or a wad of leaves, or even solidified chemicals like cyanuric acid or borax. It could be a collapsed (or collapsing) pipe, or a failed valve.
If you want us to help you'll need to post pictures on Picasa, Photobucket or something of the kind (no porn hosts, please!). But, it's still hard to solve this sort of problem over the internet.
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