You have to perform a series of moves to make it pickup a toolpath and start cutting. A lot of it depends on if you are running with a THC controlled by MACH. Here is what I do:
Note the g-code line you stopped on
Scroll backwards through the code until you find the beginning of that cut (hint: it's the M03 to fire the torch on that cut.
Jog the machine over close to that spot and do a Z ref or a manual zero on the top of the material
If you have a MACH based THC turn off the THC button in MACH
Find the line before the M03 that moves XY to the pierce point and use the "Set Next Line" button.
Hit RUN and as soon as the torch fires turn it off using the screen button.
Let the torch move along the toolpath until you get to about 3/4" from where you want to pick up the cut THEN turn on the Torch
Once the torch start cutting new metal turn the THC button back on.
It sounds complex but once you do it a couple of times it gets easy