Several attempts resulted in failure on a network where Tor used to work. I noticed that DuckDuckGo and Startpage and Ixquick used to work also but now get an error message when access is attempted. My only conclusion is the system admins must have noticed the use since regular access to the search engines stopped about the same time Tor started having problems on the network. How do I determine if Tor addresses are getting blocked? Do I have to use a bridge in this case?
Yes, you need to use a bridge in this case and if they are blocking Tor-traffic in general you also have to use the obfsproxy which is scrambling the Tor traffic so it looks like normal HTTP traffic. Checkout obfsproxy site on torproject for more details.