Every litre of water you use has to go somewhere, and with a blocked drain that somewhere is the lowest opening — usually a ground-floor toilet or shower tray.
Do not attempt to plunge or rod a system that is already overflowing, and do not pour chemicals in. Both make the engineer's job harder and can make the spill worse.
If you can safely lift a chamber lid and see whether it is full, that information helps the engineer arrive prepared — but never lift heavy lids without proper keys.