Not printing at all. Screen appears as series of exclamation marks

I had the same problem with my label marker. Changing the batteries fixed the problem

I JUST HAD THE SAME PROBLEM YESTERDAY. SO I TOOK MINE APART AND FOUND THAT ONE OF THREE SMALL WHITE DRIVE GEARS WAS BROKEN. I'VE HAD IT FOR 6 YEARS BUT ONLY PRINTED ABOUT 20 LABELS. I CALLED TECH. SUPPORT AND THEY SAID IT WOULD COST $70.00. I ONLY PAID $90.00. TIME FOR A NEW ONE

This is probably due to a broken nylon pinion gear, the one which meshes with the capstan drive motor. Due to poor design, the gear is only marginally heavy enough to stand the strain of being press-fit onto the shaft that runs through it. The failure mode is that the gear splits along one of the seams between the teeth and then simply spins on the shaft. This appears to be a common problem with the LabelManager 300, which is really too bad because the printer is otherwise an excellent machine. Anyway--
Replacement parts are not available, and the design is a ***** to repair but it can be done. The short story on the repair trick is to:  1. unscrew the two fasteners (one near the print head accessed from the front and one accessed from the rear once the back is off the unit) that hold in the motor and gear assembly, 2. pull off the motor worm gear 3. unclip and push motor back
4. remove the mating gear/shaft assembly by extracting the shaft running through (broken) mating pinion gear and associated 2nd stage worm gear 5. clean gears and shaft with pure iso alchohol 6. drill out split pinion gear with #47 (.078") bit 7. superglue pinion gear back onto shaft, centered on spline (i.e. where it was before). Use pliers or clamp to hold gear snugly against shaft for 12 sec or so while glue sets 8. press fit 2nd stage worm gear back on the other end of the shaft (again, centered on spline where it was in the original assembly) 9. modify motor assembly plastic baseplate by cutting narrow "V" **** in plastic "tunnel" that shaft previously went through, so that shaft can be pressed into place without disassembly/reassembly of gears. This is the trick... 10. when shaft assembly is back in place, reassemble worm gear onto motor shaft and press motor back into position. Worm gear should now mate with pinion gear on shaft assembly and the motor shaft should be readily turnable with fingers. 11. reassemble motor/gear assembly onto label printer metal baseplate and test.
Be certain, for Step 7, that the gear and shaft mating surfaces are grease-free and use a good-quality superglue. Epoxy does not work for this repair!!
Have fun and good luck. crd

According to the manual:
No response when pressing the print key
!!!!!!!!!!!!!!!!!!!!!!!!! in the display indicates a printer jam. Press any key to return to normal display, then, open the cassette compartment and free the jam.
I had the same problem but there was no jam so I haven't been able to fix the problem. Even removing the original tape cassette with a new one doesn't fix the problem.