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Hello, the water leak was most likely caused by the boilers pressure relief valve releasing pressure, that is normal and not a problem, it will adjust itself, there is no need to add water again unless the boiler pressure gauge goes below 15 psig or 1 bar.
The only problem that you most likely have is a dead battery in the thermostat wall unit.
This should be the boiler manual I am parking it here for future reference is needed.
I need the brand and model number of the wall thermostat that is showing the bad battery symbol, the front of it should pry off on most models, with the battery inside, and the full 12 digit model number inside... some photos will help a lot.
Use the paper clip icon above on the right end of the tool bar to attach the photo's.
We can go from there.