I let an XCode project sit unchanged for a while, and when I got back to it I was getting the following error:

No such module 'AlamoFire'

This error was happening for all of my pods, not just Alamofire. There was some conflicting advice on the solve, but this is what worked for me:

$ cd ~/my-project
$ pod deintegrate
$ rm -rf MyProject.xcodeworkspace
$ pod install

This reinstalled the modules and built my project successfully. Hope it works for you too.