As others have said, this is very common. And the main use I get for my staking set. (Ã°ÂŸÂ‘ÂŒÃ°ÂŸÂ‘Â� clockbloke!)
If you do any regular repairs then this is a very handy and worthy tool to have.
That said, it is very easy to find a method for doing it without the staking set. I believe I had done it by placing the axle In the crook of my tweezers and using a hand setting tool (with small hole) to firmly set the wheel back onto the assembly. Check with a loupe that the two are completely mated.
I always like to reduce the action if the reset, and especially so when this has happened.
I have had quite a few busted minute recording wheels, also sometimes the end of the axle gets snapped off where it enters the chronograph bridge bush. It is my view an over strong hammer click can be destructive and I always tweak this component as instructed in the Seiko maintenance sheet to give a light click action. The hammer works without the click so I usually back it right off.
I do not have a staking set but have had success refitting the wheel to the axle as described by Tom as the fit is still good enough to resist the reduced hammer weight and stay put. I place the wheel on a wood surface with a small hole under to allow the end of the axle to poke through the wheel and press the axle in place using tweezers or small snipe nose pliers lightly grasping the axle and pushing the shoulder on the axle. Make sure the axle is fully home in the wheel or the assembly will not rotate freely when installed.