Thoroughly wash the electrophoresis equipment. Rerun the experiment. If this fixes the problem, then the equipment or the technician's gloves may have been contaminated with an enzyme that degraded and the product, or another substance that inhibited the reactions.
If the experiment fails again, run another electrophoresis gel using different electrophoresis equipment. If this fixes the problem, then discard the faulty equipment.
If the experiment still does not work then rerun the experiment with different reagents such as buffer, polyacrylamide, dye and so forth. If this fixes the problem, one of the electrophoresis reagents may have been contaminated or otherwise faulty.
Repeat the procedures -- PCR, bacterial amplification, etc. -- that yielded the DNA sample in the first place, if you are getting a blank gel. If you are getting bands of the wrong size, then redo the digestion steps; for example, use restriction enzymes to isolate the desired DNA fragment; splice the receiving plasmid, if any and ligate the products. If this yields the correct results, then the problem was with the procedures that produced the DNA sample; not with the electrophoresis gel.
Repeat the experiment using uncut DNA. If this produces the predicted results, then the problem was with the digestion enzymes or procedures.
Repeat the procedures such as cell lysis, and so forth, that yielded the protein sample in the first place, if you are getting a blank gel.
If you are getting protein bands of the wrong size, then redo the SDS steps, being sure to heat the sample to allow the SDS to bind. If this yields the correct results, then the problem was with the procedures that produced the sample, not with the electrophoresis gel.
Repeat the experiment without the SDS. If this produces the predicted results, then the problem was with the SDS or associated procedures.