A few...
I trust you have verified that the flash fully powers up and you get the red pilot lamp showing it's ready to fire.
On your 60D menu system, if you find the "Flash control" option, you can make sure you haven't set the external speedlite to "disable" -- although I think in that mode you wouldn't get the message you're describing. i.e. I doubt this is it but you may as well double-check.
A more common reason for failure is that the flash camera can't fully communicate with the flash. This is usually because of a bad connection in the "foot" of the flash.
The flash needs to be fully seated in the hot-shoe rails. Make sure the lock level is released, slide the flash fully into the hot-shoe rails until it stops. Engage the lock on the foot so it's held securely (it will will slide to the right and then click-to-snap (you have to press the tiny button to release the lock lever). This doesn't *just* snug the foot... it also drops a tiny metal locking pin into a small hole in the hot shoe so that the flash cannot slide out.
At that point, the "foot" of the flash is actually the "ground" connection and it's in contact with the metal rails of the hot-shoe AND all five of the contacts on the foot should be in contact with the camera.
This assumes there's nothing wrong with the flash -- and there could be something wrong with it. So if you're confident you've fully seated the flash, then remove the flash and inspect the "foot" on the flash.
The metal plate on the foot of the flash should be solid... if you use your fingers and attempt to wiggle it (don't use heavy force... we don't want to break it -- just grasp it firmly and see if it will wiggle at all) it should not move. If it does move it means that the foot has come loose and is not secured to the flash body (there are four very tiny screws which hold it securely but they can only be accessed from the inside of the flash.
Next... inspect the 5 metal pins on the foot of the flash. Each of these pins is spring-loaded so that they'll press against the 5 contact points in the camera hot-shoe. I have occasionally seen a spec of something (e.g. a bit of sand) get wedged in and "jam" a pin so that it it no longer makes contact with the contact on the hot-shoe. If you view the foot of the flash from an edge-on view, all the pins should spring back out to the same height. If one of them is recessed lower than the rest... it's jammed.
The 430EX II has a solid reputation as a work-horse flash. It tends to be among the more reliable flashes on failures are rare.
It's also possible that the problem is in the camera. An easy test of this is to try another speedlite (if you have one or if you have a friend who has one.) If it is your speedlite, and it's under warranty, then it's probably a simple remedy by Canon service.
If the flash is not under warranty and the problem is with the hot-shoe connection not being solid, then it's often an easy repair. If a foot were irreparably damaged, the entire foot can be replaced (the whole thing is a module) and it's an easy and I believe somewhat low-cost repair.
Tim Campbell
5D III, 5D IV, 60Da
