What do I do when I am involved in an accident or collision?

Contact authorities (if needed). Contact Uber.

First of all, you should call the local authorities to report the incident.   This is especially true if there are any injuries or you require emergency help.

You also want to contact Uber as well.  To contact Uber, click here: 

https://help.uber.com/h/3ce56ca8-4cd3-4df5-9953-1b464001d36e

UPDATE:

No, there is no longer an emergency (800) number for Uber according to this article by Verge:

The number (800-353-8237 or 800-353-UBER) was first reported in Quartz, which found it buried several links deep in the app. And when Inc. followed up, an Uber representative said there was no emergency contact, only to walk that statement back the following day. The hotline had been kept under wraps to test for its "discoverability" by customers, Uber told Inc. It's unclear why Uber would create an emergency hotline and then not tell anyone — though Uber claims that users who stumbled across the number would call for customer assistance.

Source: https://www.theverge.com/2016/3/10/11196352/uber-secret-emergency-hotline-panic-button-kalamazoo

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Uber Driver, CEO of TaxiFareFinder and Unleashed, LLC