When you open up the pipes to do maintenance on the boiler, at least part of it ends up getting drained. A little bit of air trapped in the pipes is all it takes to block the flow. The most likely problem is you need to bleed the system to get the air out. Now, sometimes, that can be a real pain. THe air will tend to accumulate at the highest point. FWIW, the circulator pumps used on most systems are NOT big enough to push water up very high...they work fine if there's no air since the water falling down on the return helps, and the pump only really needs to overcome friction, but it will not work if there's air.
Time to bleed the lines...how best to do that, depends on what's available, and we cannot see that.