There will be a number of pre-checks performed by the washer before starting.
It sounds like there is a aqua sensor which is giving a fault, and stopping the programme from starting and locking the door.
This is likely caused by a build up of water in a compartment in the washer, which may be been caused by when you cleaned the pump.
Firstly I would empty all water from the machine by removing the pump filter as described in the manual (if you need directions please let me know), this should remove all water from the machine, with the filter removed and hatch still open, I would then try and turn on the machine and see if the door mechanism locks, and programme starts. If it does be ready to switch the power off at the mains, as the water will start to flood on to the floor out of the hatch, at this point you can then replace the filter and start using the machine as normal. If the machine still fails to lock the door and start the programme, there is a fault being detected by a sensor, and this will need further investigation.
I would suggest you call out an engineer in the event of the a sensor fault, as it requires a good understanding of the wiring loom and what each sensor looks like to test its output with a multi meter. If you wish to go down this route please provide me your post code and I can recommend someone.
Please let me know how you get on, or if you need anything clarified.
Its an interesting one, have you tried draining the water out (again) via the filter, with the machine switched off; it may help to tip the washer forward by 20 degrees to help fully drain it, put the machine back in place, then turn the washer back on and let it complete a rinse cycle without interrupting it?
When the display is showing "YES" it means the unit is in a safe state to empty or add laundry (the door should be unlocked), so if its still showing when trying to start a program it sounds like the fault isnt generating an error code, so it will defiantly require an engineer to troubleshoot.
Please let me know if I can be of any further assistance, I would appreciate if you rate the service received today.