Just based on your post alone, it is hard to identify the problem, as it could be anything.
First and foremost, I recommend you take a backup of your current site (including the database). If you don’t know how to, ask someone else trustworthy, or the company that hosts your site.
Now, as for why this happen, there are many possible reasons, such as :
- Malware
- Corrupted database
- Plugin issues
Until there is more information provided, it is hard for me to say what the issue at hand is, and how to go about rectifying it.