Need some additional information:
1. What version of El Capitan? The last one was V10.11.6.
2. What exact Mac model do you have?
3. Have you ever done any disk cleanup/maintenance/repairs, from a software perspective? You actually can do a "fair to good" amount of disk cleanup yourself, and there are some excellent products available (both free and commercial) that can help you.
4. Are you making backups to an external device?
5. Prior to the issue happening, did you do any software updates to any software you have?