Our hot water immediately stops flowing in the entire house after the toilet is flushed. It eventually returns at full pressure but it is not related to when the toilet finishes filling. When the toilet flushes there is also a loud banging in the system.
I have turned the water off at the main valve , drained the system, and then turned the water back on because I thought it was "water hammer" but this hasn't helped.
The problem started during a recent cold snap. I don't know if this is relevant.

Remove toilet tank lid. Flush toilet. Does bang and lack of water occur when tank begins to fill or when tank is filled and fill valve shuts off?