If it is an Ecotec plus, there are two grey handles underneath the boiler one on the left and one towards the middle, both will read 'closed' on them as you look at them.
Open both slowly (1/4 turn), the water will start to enter to re-pressure the boiler. while watching the guage (right hand corner of the boiler), until it is into the grey area, then turn BOTH back off.
Press the minus ( - ) button under the display, this will give you a reading in terms of bar pressure as to how much pressure is in the boiler, once it is reading 1 to 1.5 bar you are where you should be.
Then press the boiler reset (above the minus button) and things should return to normal.
Carefree, February 2011