It's always a good idea to re-beat the buttercream, it helps it regain it's smoothness. And yes, you can use a hand mixer or even a large spatula or wooden spoon. Just get it remixed and you'll have better final results.
Oh and Welcome to CC!! 
I suppose you don't "have to" but it just whips it back up to a nice consistency. I'm sure you could just use your hand mixer - does the same thing as the KA. Plus, it's not like making frosting which the hand mixer would take a beating (no pun intended) You are just mixing
Have fun!
it probably also depends how long it's been in the frige. If only a few hours, then it just really needs a good stir. Longer it may dry a little and need more mixing and even a few drops of liquid. Just check the consistency to make sure it's how you like it before you start decorating.
Yes you can re-beat the buttercream. If it has been sitting too long in the fridge, you might need to add more drops of water until you reach the desired consistency. For more information on buttercream consistency check my blog: thecakestylist.blogspot.com
I just re-beat right after I let it sit at room temp. for a few minutes and I have'nt had a problem at all. Good Luck!!
Quote by @%username% on %date%
%body%