I had applied for a company that does managed IT services so the question is quite fitting. It was asked by an HR recruiter who probably wasn't very technical herself.
I have trouble with questions out of context like this. For example is it assuming you know what is causing the problem but don't know how to fix it? In IT support most of the time you don't know how to fix something at first glance (i.e. you can't be 100% certain just from reading the comments in the ticket) so in a sense most of the time "I don't know how to fix something" but I'm guessing this isn't what the question is asking.
I ended up saying "I would do a Google Search or consult the internal knowledge database or documentation, depending on what the problem was. If that didn't work I would reach out to a colleague who may have more knowledge in the area." Was this good? Was this the kind of answer they were looking for?