Recently government banned couple of sites (including blogspot.com, geocities.com etc). Government send noticed to more than 150+ ISP to block these sites. Now these sites were not accessible from India (since last Friday).
Since I work at one of the leading Indian ISP, we started to receive call from angry users. This is the one of those time, you need to tell the truth immediately about the problem.
We provide them a reasonable explanation of why sites are blocked. Most users appreciate this. We told them it was not our fault we are just following governments order.
I think one need to always tell the truth. For example, network is down because of router crashed or server crashed etc. Never ever drop bomb on your user. Users are smart enough to find out exact cause of problem these days :)