No power
Check the switch, the interlock and the fuse. If the machine is inside an enclosure, confirm the door interlock is closed.
Weak or uneven beam
Clean the lens, re-check focus and confirm the material is flat. A dirty lens is the usual cause.
Misaligned cuts
Check belt tension and that the workpiece has not moved. Re-home the machine before the next job.
Software will not connect
Try another cable and port, and confirm the driver for your model. Note the error text before you reset.
When to stop
Cut power if you smell burning electronics, see a spark outside the work area, or the machine moves without a job.