Crack in header on intake manifold

I had brought in my vehicle into the shop for an oil change and for them to swap out my thermostat. ever since I pulled it out of the shop it was idling low it calms down when I take off it takes a second to catch up to the right gear. I ended up noticing I took off the face plate of the header on the intake manifold and there is a crack right in the middle of the header. It was not like that until after I had gotten it out of the shop and noticed that there was no coolant in the reservoir. I had put a whole jug of coolant in the reservoir by the next day the reservoir was bone dry. then I had put half a jug of coolant into the reservoir which topped it off and the coolant level has stayed. Could this have been a I thought the problem the shop caused. For a quick fix temporary solution would J-B weld be a good idea as a temporary solution until I can replace it?