This no accommodation at the lagoon itself but there are several hostels and other accommodation options in the surrounding area. There is also accommodation at Höfn, 60km east.
The most convenient hostel for the lagoon is Vagnsstaðir, 20km to the east and a member of Hostelling International. It can get fully booked in summer so try to reserve in advance. The nearest shop selling food is in Höfn, so be sure to stock up there or bring some from elsewhere.