No bands on agarose gel after restriction digestion. - (Apr/05/2011 )
to test if your enzymes are contaminated or the DNA is you could use another DNA prep or even lamda DNA which has similar restriction sites and trying digesting them. If you see smear with that that means the enzymes are contaminated. If you see it only in your DNA prep that means the DNA is contaminated. Best way would be to prepare a new preparation or try to phenol chloroform the old one and see if you get any bands.