dash hashrate is zero. Why?

The pool was just restarted for a software update. When that happens your hashrate resets to zero until it is recalculated roughly 10 minutes later. It's all in the FAQ that everyone is encouraged to read.

I have read and understand the FAQ.   It does not say anything about the pool restarting and resetting stats.  Is there a way to tell when the pool restarts or how ong it has been running?

Pool is restarted by the pool owner whenever he needs to do so to apply any source code update, so it's rather random and miners are usually offline only for couple of seconds before reconnecting, so the impact is minimal.

A pool restart takes only a fraction of a second is not even remotely comparable to a server reboot. Shares are buffered in between restarts.