Woke up this morning to a cold house and no hot water. Looked at the boiler and there is a light flashing showing it has low presure. This has happened once before and you have to open a tap on the underside of the boiler unit to presumably let more water into the system?? Tried it today though and nothing happened as in you cant feel anything flowing as you open the tap.

Can any plummers advise me as to what this might be before I call someone out.

Cheers for any help.