This happens every year to my 1st floor zone when it's in the single digits for more than 24hrs. -- that zone doesn't get used due to the coal stove so it freezes solid. The only way it thaws is if the temp gets up in the 30's for at least a day, & I manually open that zone valve & rap on the pipe with a hammer in all the corners of the house. Once it freezes, it's tough to thaw......unless you have 3 friends with heat guns!
I'll bet the same thing happened here.
Your high limit shouldn't be above 180*-190* . I've found that when I set mine to 200* or above I get MEGA DHW & the rooms heat up quick, but it uses almost 50% more oil! Not worth it IMHO. When my coal stove is off for the season, I set mine at 180* HI with a 10* diff. (most aquastats have no diff adjustment for HI), 150* low with a 10* or 15* diff. Keeps my indirect hot water heater hot & doesn't burn too much oil.