The economic downturn has put a Band-Aid on one of the most vexing problems in healthcare: a shortage of nurses that has slowed care at some hospitals and forced others to turn away the ill. With some nurses postponing retirement and others resuming their careers for financial reasons, many hospitals across the region and the nation say they have few, if any, openings. After more than a decade when hospitals struggled to maintain sufficient staffing, the want ads have virtually disappeared, and only acute-care and emergency-room nurses remain in great demand.