No pressure for boiler

Jack_7

I cut the pipe in order to add one more baseboard to the loop. The pressure gauge goes down to zero and never come up when the boiler firing. I also stupidly press the valve on the top of reseve tank which had pre-charged 12 psi. The air was gone from the reseve tank. What can I do to get the presure back? Is the reseve tank no mre good and has to be replace? The boiler is running good but there is no heat in the rooms.