Just because a post leads to a service or external site does not mean it is inherently bad, so you don't necessarily need to flag it. We don't like link-only answers (partially for this very reason), but they exist and don't need to be deleted on-sight all the time. Check out Shog's answer here for some discussion of how to handle link-only posts, especially some of the other posts here linked too.
But when the link breaks (as is the case here) then the post offers zero value and needs to be addressed:
- Does the service still exist with a different URL? Then edit to remove the broken link and replace it with the new link (and perhaps expand the answer to offer some more description of the tool/service and maybe even how to use it).
- Is the tool/service still available, but extremely outdated and only works on old version of software? This is a gray area since it might be still useful to someone. A comment on the post is probably best, maybe editing in a brief blurb about the compatibility limits if the OP doesn't respond to the comment.
- Is the tool/service completely gone or non-functional, then the answer offers zero value and needs to be deleted. Flag away. A custom "Other" flag works best here and make sure you clearly explain what is wrong (for example, "link points to a dead site and without it the answer has no value").
I intentionally did not mention downvoting above at all, as that is a separate issue. Whether to downvote (or vote in general) or not is entirely up to you. Did you find the answer unhelpful? Then downvote. If you didn't, then don't. Personally, I don't see a link-only answer with a broken link being helpful anymore, so I'd downvote it, but that's just my opinion, you or someone else might disagree.