Welcome to iRV2! It would help if you put your rig in your signature so that folks that have a similar setup can jump in to help. Generally, those seals are either attached with an adhesive or have metal teeth that grip a ridge so that they stay in place. Best thing to do is crawl under the rig and determine what's going on. The seals are relatively generic and can be found at most RV parts places. You may be able to fasten it back in place, but my experience has been replacing with new to be better.
On edit, I assumed the seal was hanging under the slide on the outside. If the seal is under the slide inside the rig, that might be a little more complicated....
Dave & Jo Ann
2008 HitchHiker Champagne 35LKRSB GONE | 2011 F350 Lariat 6.7PSD GONE | Many great memories!