If the printer isn't turned on or selected, and is yet still connected to the port or card, then the modem won't work. The program sends an initialization procedure to the printer, and if it doesn't get something back from the printer firmware, it will hang.
Usually what happens is that a "@" appears in the upper left corner of the screen.
NOTE: This same problem will occur if the printer interface isn't on an Apple Super Serial Card: AppleTerm will only acknowledge an Apple SSC.