Hot and cold water stopped flowing in shower at same time
If you are on a municipal/utility water supply I'd guess that a valve in the system was turned off, and then it (or another valve, if they were bypassing a section of pipe) was turned on.

If the problem recurs on a frequent basis, that may not be it. For a one-time or infrequent event, that's my best guess with this little detail to go on.