Most BMW's suffer from a loss of sreen wash jets. Follow the following procedure to cure this fault.
1. Top up the washer bottle, you never know it may just be empty.
2. Check the washer pump fuse, it can often be part of the winscreen wiper circuit so check that too. If a fuse is blown replace it with the same amp fuse (dont over amp fuses)
3. Locate and remove the washer pump on the washer bottle (locations vary). Once this is removed take out the rubber lug that will be left in the bottle (it may even come out with the pump). Attached to this will be a filter, this may be clogged with grime, simply run it under the tap and stab it with a nail brush to remove the grime.
4. Remove the connector from the washer motor and check for 12v using a multimeter. If 12v is found replace the motor, if not look for a broken wire.
Beyond the above simple checks and repairs, contact your BMW dealer or an auto electrician.