On the left hand side, under "My Downloads", you will see "Downloads." Here you have the ability to re-download a previously downloaded song up to two more times within 2 weeks from the original download for reinstall purposes, in case your connection timed out. This re-download will not count against your download totals.
In the mobile apps there is no Recent Downloads section. If a download fails, there is a feature built into the app to recognize an incomplete download, and the next time you open the app it will ask if you want to resume the downloads that have not completed.