I give my females a full 24 hours to lay their clutch before checking to see if there is a problem, mostly being egg bound.
Occasionally, a female will lay 1 or more slugs up to several days prior to laying the rest of the clutch. Is the first egg a slug or does it look fertile? If she hasn't laid any more eggs after 24 hours, check to see if she is egg bound...a very noticable egg lump will probably be seen at her vent. If so, get her a herp vet soon if possible, or try the methods of dealing with egg bound snakes outlined in Kathy's book.
There is also the slight chance that your female only had one egg to lay. I have had some very small clutches, but must admit, never just one. (Did have a clutch with only one fertile egg this year, but she also laid 5 slugs)