There aren't any lounges that I can think of that'll be of any use to you. There are a couple of options for day lets for hotels that'll at least give you a place to crash.
The Sheraton at the FRA does day lets, but isn't cheap. The other options are the NH Hotel just off site, but with a free shuttle,
Hotel reservations, find and book hotels NH: Madrid, Barcelona, Amsterdam…, or the InterCity Hotel, which also has a free shuttle, and is the cheapest option.
InterCityHotel Frankfurt Airport. It's pretty basic, but clean and convenient.
If you do book at the Sheraton, book on the club level so you can have access to the lounge, the food is very good and the selection of drinks is also pretty good. If you book at the cheaper options, and if budget is an issue, there's a little supermarket in terminal One, I think on Level 1 at the far right of the terminal as you look airside.
I spend far too much time hanging out at FRA!