Expected bands in polyacrylamide gel
If your restriction enzyme only targets a single site on the DNA sequence then it will split each DNA strand into two fragments. If the restriction enzyme cuts every strand then you will see those two fragments. Note that if the two fragments have the same length they could appear as a single band when separated using gel electrophoresis. 

If the enzyme doesn't cut everything the uncut DNA will also be present. This is why you should always separate some uncut DNA in the gel electrophoresis.