No digested product bands, but markers are visible. What could be the reasons?
Usually there is no issue of mixing. One possible reason is degradation. But you say marker is faint (marker wont get degraded). Stain the gel with extra EtBr and see again. If you reused the gel then definitely stain it again because EtBr migrates towards the cathode.

It doesn't seem like degradation to me but to be cautious for the next time:


Use freshly autoclaved water. If you are not sure then filter using syringe and keep some 10ml for yourself
Ensure that the plasticware are also autoclaved properly.
Use endo - plasmid extraction kits, if available
Dissolve the plasmid in TE (only if conc is high and you need not put much in the reaction mix else EDTA interferes in the reactions. In that case use Tris-Cl pH-7.6 ).
If you have to digest a lot of DNA then set up small reactions in many tubes.
Do not incubate the reaction for a very long time (>6h).
Inactivate the enzyme by heating if you are not running the gel immediately.
Ensure that the gel running buffers are not too old.