Hi Cara Joos,
WhatDavid Pike's mentioned above is right.
However, if you are after tools out of the toolbox, you can use Dissolve tool for the polyline provided that it has a field with the same value so that you could use it as a dissolve parameter/field to aggregate features. you can add a field with same values if it's not already there.
Once the polyline feature class is aggregated (continuous), use the Feature to Polygon tool.
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Note that sometimes just using the Feature to Polygon tool will do the split without using the Dissolve tool, depending on your polyline feature class.
I hope that's helpful.
Cheers.