I use a little batch file to turn off my secondary monitor, which is really the big-screen TV in the adjacent room, but I don't necessarily link it to another program. The line is:
"C:\Program Files (x86)\Dual Monitor Tools\DisMon.exe" -1p -2d -x
How do I go about starting that second monitor up again?
DisMon can only re-enable a monitor that the same instance of DisMon disabled. If you run DisMon with the -x option, then this causes it to exit and not restore the monitor states.
So in short, in your case you cannot use DisMon to re-enable the second monitor. This is something I will address when I rewrite DisMon.