Short answer: the editor will continue to try and find other suitable reviewers, sending them invitations and waiting for replies. At some point, they may come back to you and ask you to suggest names (additional names, if you had already given some when you originally submitted the paper).
It can happen that an editor has a hard time finding suitable reviewers that accept to review a manuscript. There are a few factors in play, such as:
- the journal is not well-know;
- the editor is new, or not well-know in the field, or does not have a very good network;
- the research reported is atypical, in a very narrow subfield, or joins different areas (so that no one reviewer feels confident in accepting)
- the elements sent by the editor (title and abstract) are rather boring or tend to confuse the potential reviewers
Note that most of these factors do not reflect badly on the manuscript, so there is no need to feel bad about it. The editor is likely to continue his search of reviewers. I see two other options that the editor could choose from, but I judge them as rather unlikely:
- Evaluate the manuscript himself, and make a decision based on his own review. After all, it's his job, and the reviewers' role is only to help the editor reach a decision.
- Refer the manuscript to another editor, or to the editorial board, so they can make a decision on it.
Finally, note that it may take quite some time to find suitable (and willing) reviewers. In the case of one paper of mine, it took the editor 3 months to find adequate reviewers, and that was actually for a prestigious journal. However, the paper was atypical enough (and the research was quite novel) that many potential reviewers did not feel able to review it adequately. (And in case you wonder, it was accepted on the first try, once the editor found reviewers.)
Edit (regarding your comment): my advice is don't retract your submission, unless you think it's the journal's fault (unwilling editor or unknown journal). I know it's tempting! But especially if your work is multidisciplinary, it will take time to find reviewers, even if you submit it to another journal. Thus, better let the current submission process go to its end.