I'm no expert, so others may have better suggestions, but my first thought is that the toolpath display in Mach has two modes. In one, you see the toolpath relative to the entire machinable space. So, if it is pretty small relative to your machine it will look like a dot. There's a button to toggle modes. Try it. The other mode shows the toolpath scaled to fill the display.
In the Toolpath screen (the one where the toolpath picture is bigger) you get to see the X, Y and Z extents of your part file. I always check that they are correct. By the way, in that screen, it can be important to press the button to redraw the toolpath as it may be showing a previous version if you've hand edited the GCode. In fact, I'm surprised you tell Mach3 to go ahead when you can't see what it's about to do in the toolpath window. Rather trusting of you.
In any case, if you click in either of the toolpath displays you can then use your mouse wheel, if you have one, to zoom in, the right button to pan, and double left click to reset scale if you zoom in too far.
I presume SheetCam is outputting valid GCode, but if you hand edit it you may introduce errors that can make the Mach3 toolpath display disappear. But in that case I don't think Mach3 would actually machine the part, and it's usually pretty good about telling you in the Status line where the error is.
Things get harder to analyse, in my limited experience, if your main part file calls external subroutines or macros that you've written, as the error may be in the subroutine or macro file.
By the way, the arrow in the toolpath display is showing the Z axis going up and down. If the display is showing X-Y then Z moves would otherwise be invisible.
Hope this helps
Chris Lusby Taylor