PDW is parallel data warehouse. It's a huge appliance that your company paid a ton of money for. Read about it here. Polybase is a way to query other database engines, Oracle and a bunch of others, directly from within SQL Server. Read about that here.
As to understanding what happened on your system, since you're posting in the 2017 forum, we can take advantage of the system_health Extended Events session. Read about it here. It's going to show you a bunch of the stuff you need. What was hung, why, etc. However, it's not easy to dig through, so get ready to learn how to use the tools & data.
Other than that, I'm with Jeff. I can't see your servers, logs, etc., so it's really hard to make any concrete suggestions.