Boiler lock out since turning thermostat up

Hello,  Our Powermax 155x had been working fine until we turned the thermostat up last week.  It had beeen effectively off/turned down low during the summer and so was the first time the heating was required.  The next morning we woke up and the boiler was on lock out.  We have turned it off and on, but still the lock out light comes on.  The pressure was just below 1 and so we increased it, but it still locks out.  Any ideas why as it only happened when we tried to use the central heating again.  We currently have no heating or central heating due to the lock out.  I would appreciate any help you could give.  Thanks.