There ya' go--ask a pertinent simple question! Atta' boy, haller... :) To OP, that's a sidebar joke... :)
If it's relatively large, I'd start by trying to fish out what can be done from either/both ends and if it isn't totally plugged follow that w/ a pressure nozzle on a garden hose. Often that will be sufficient. If the grade is such that there's reasonable access, a cobbled-together extension of 1/2" copper pipe on the end of the hose can be used as a combination ram and water cannon. A "nozzle" as simple as simply flattening the end w/ a hammer can be quite effective.
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