Our trusty dishwasher (Bosch Exxcel) had recently been getting worse at the daily job of crockery cleaning, and today it simply did no cleaning at all! On starting the wash cycle there was no water coming out of the cleaning arms, and the "check water" light had come on. To fix this, I did the following (after turning off the water and electricity supply to the machine):
Remove the top cover. This is held in place by clips at the back; use a flat-head screwdriver to prise each one up, and the top should then slide back a couple of centimetres. You can then lift it off.
Remove the left side panel. There are two torx screws on the front of the dishwasher; undo these, and the panel should pop off.
Looking at the left side of the machine, you'll see a soft-plastic pipe system. Near the top is a float devices. Ours was surrounded by brown gunk, so we removed the unit and gave it a good old clean with lots of water and detergent. It's very difficult to get at the section containing the gunk, so we kept shaking and gently banging it until most of the gunk had come out. NOTE removing this plastic pipe unit is straightforward, but there are several pipes going into it! Keep note of what goes where, and you'll need pliers to remove the circlip keeping the water inlet hose in place.
Clean out the pressure chamber. This was completely full of slimy brown gunk, and was the main culprit for a lack of water supply. Looking at the left of the machine, it's at the bottom and near the right. It has a micro-switch and a hose attached; both of these need disconnecting before pulling the unit out. After cleaning, don't forget to replace the red rod in the correct place, as otherwise the machine won't know when water is flowing!
I strongly recommend taking photos of things before you take anything apart. You can see from my photos and video below that there are lots of pipes and a bit of electrical componentry visible, so it would be easy to confuse things. If you don't feel confident, just stop and get a professional applicance repair technician in. Whatever you do, you do so at your own risk! The information here is what worked for me - but it may not work for you!