You have been working on a project for a few months at work, and it's almost finished. Your coworker just told you that a high-level director in the company wants to change part of the project, which means that you will have to re-do several weeks of work. You're annoyed and you can't believe this is happening. You say this in frustration.
Asking "Seriously?" is similar to asking "Really?" There's a bit of a difference between them, though. You ask "Really?" when you're surprised by something. When you ask "Seriously?" it often means that you're surprised, but also annoyed or disapproving.
As another example, imagine that your sister just told you that she was dating someone who you think is really unattractive and has a bad personality. In this situation, you might ask:
You should also note that you can use the word "seriously" in a complete sentence. For the situation described at top, you could ask:
Is he seriously going to make us throw away the last two months of work?