Info on Frankfurt airport

It's too late to change your tickets, I imagine, but Lufthansa (United partner ... ) has superb Business Class lounges, and even better First Class lounges. I wouldn't count on anything from the airline for Economy passengers. Sorry, I don't know about general lounge or service availability.

https://www.frankfurt-airport.com/en/flights-and-transfer/leisure-activities.html
https://www.frankfurt-airport.com/en/airport-guide/eat-drink.html

With that kind of a layover, go into the city. There are a wide variety of sites to see, depending on your interests. The train into the city only takes 15 min. and it runs every 15 min. Near the city center (6 stops from the airport) you can see the new old town, 3 different medieval churches, archeology, Jewish heritage sites, the wonderful Klein Markt Halle, and depending on the day of the week, one of the great farmers markets. Walking around in the sun, will help you with your jet lag.

These are some events happening in May:
Frankfurt Fressgasse Fest – 17-26 May
Frankfurt Paulskirche Fest – 18-21.05
Frankfurt City Forest Fest – Wäldchestag – 26-30.05
Frankfurt Pealing of the Bells – 27.05 16:30

so sorry that happened, as it's really frustrating after an overnight flight .
There are two hotels actually in the airport , I think it's Sheraton and Marriott. I'm not sure if they have a day use rate for a shower and nap. There are business class lounges, but I'm not sure they have passes for purchase. What is your airline? Do you have a Priority Pass card from Chase Sapphire visa, or another credit card? I know these changes are frustrating.

If you decide to stay in the airport, check out sleepinginairports.com. My recollection from 5 years ago is that there are nice paid showers, although I don't recall which terminal (but it was international). It's a big airport.

There are showers at the airport, in both terminals.
The lounges have showers too.
For hotels, there is the Hilton Garden Inn, Marriott Sheraton, and the Hilton. There is also one called the Cloud that is airside, but for me, way over priced.

I know the airport really well as am out there all the time! Ask any questions you like.

Why give up the chance to see a bit of German history, have a good meal, be in the sunshine?

Great advice from everyone. Sounds like a lot to see and enjoy in Frankfurt. A little nervous about going out, leaving the airport and returning through security/customs. Is that a lengthy process? We don't want to be stuck in a long line and miss our 5pm flight to Florence! Maybe we'll get lucky and they will add our original mid-day flight back:)

I first flew into Frankfurt in 1996. I hated it. I thought it was an ugly, impersonal Cold War-era airport.

Then I had layovers in Frankfurt in 2004 and 2015. Now it's among my favorite continental airports.

Why? Food. I have had great meals in the Frankfurt airport. Usually, I'm not obsessed with food while traveling nor am I a foodie, but I noticed the upgrade in food offerings.

If you decide to stay in the airport, the meals -- including the fine German beer -- are excellent.

20 years ago, we had an 8 hour layover and took the subway into the old town. A group ticket for up to 5 was cheap and we had our seat mate join us for a Gay Pride March and festival food.

Sounds like some great possibilities for fun adventures and food! We have lots of possibilities to check out. Always good to be thrown out of our comfort zone a bit.

The Hilton Garden Inn at the Frankfurt Airport has a day use plan. Your room is available from 9:00 AM to 6:00 PM. I checked a random date of May 23 and the room rate was 99 Euros including taxes. The hotel is a short inside walk to the Squaire complex across the street from the terminal. Plenty of eating choices in the Squaire including my favorite, the Paulaner Wirtshaus.

If you are comfortable with leaving the airport, you may want to inquire regarding this tour with Frankfurt on Foot (scroll down to "Layover Tours"). Last fall we had an overnight stay in Frankfurt of about 20 hours and scheduled a private two-hour tour. (Ms. Jo, who posted above, was our guide-she is following the Forum rule of not promoting her own business). I felt the tour was a great use of our time; it allowed us to see and learn much more than we could have on our own. The website says a minimum of seven hours is needed; having a good guide would help you make the most of your time in Frankfurt as well as allow a cushion for returning to the airport on time.