If you carry a patch kit around and don’t use glueless patches, now’s a good time to check the glue in your patch kit. The glue can evaporate even from unopened tubes of glue. The end of the metal glue tubes are usually crimped, and there’s sometimes enough of a gap to allow the glue solvent to evaporate. If the glue has been opened, you probably didn’t tighten the cap enough to prevent the glue from drying out. It’s disappointing to find a useless patch kit after flatting.
Some people I know carry two tubes of glue, one that’s open and the other unopened. If the opened glue is dried out, they discard the tube after opening the “new” tube, and replace the new with another newer tube.
Another option is to use glueless patches. These patches, which have been around for a few years, are peel and stick patches. After cleaning and roughing up the tube, you just peel the backing from the patch and apply it to the tube with no glue and no wait required. In my experience, glueless patches don’t seem to hold as well as traditional patches, but they’ll get you home.