The SkyLine at the Frankfurt Airport has broken down and needs to be repaired. Airport officials aren't sure how long it will take, but possibly into the summer months. Shuttle buses are being used now to go between the 3 Terminals. The old monorail between Terminals 1 and 2 can also be used right now.
https://www.frankfurt-airport.com/en/at-the-airport/orientation.html
This is a helpful report. At least the Frankfurt airport has contingent alternatives, unlike the Denver airport, where if the train’s not running, two of the three concourses are completely inaccessible, and a majority of would-be passengers are screwed, until proposed walkways get built. Even then, people will be in for a hike.
https://community.ricksteves.com/travel-forum/transportation/denver-airport-to-build-walkways
FRA airport was showing this as special info on top of its website since departure of German soccer team to USA last Tuesday (June 2).
Means, it is always helpful for travelers to look at airport and train station websites or pages before departure.