I would wait and see if the rankings improve. I'm very sure that Google, Bing, etc. would not penalize significantly for a slight slip up because of someone forgetting to re-up their domain name. We are all human and I'm fairly certain that the ranking algorithms take that in to account to some degree. If a site was off for say a month or two, then I could see the site being completely dropped from the rankings in order to keep things fresh for the search results. However a site being offline for a week or two, probably not that significant of a hit. Give it a few weeks and I'm sure it'll start to recover as the bots see that the site is back up. These things take a little time to get going again.
