Sorry.

"Sorry" is a way to apologize for a small mistake you made. When you make a larger mistake and want to apologize more strongly, you can say "I'm sorry for ___ing":

I'm sorry for calling you by mistake.

However, in most situations this would be much too strong of an apology. So just saying "Sorry" is sometimes more appropriate:

Sorry I didn't call you back sooner.

(someone) got sidetracked

When you "get sidetracked" it means that you were trying to do one thing, but then started doing something else instead:

I was trying to cook dinner but I got a little sidetracked.

You can say that you "got sidetracked" by something more important than the original action, or by something that's not important at all.