I have a website onlinewebdirectory.com & my website keywords rank position on Google is abruptly falling down.

My website primary keywords are as follows:
1. Web Design Company
2. Graphic Design Companies
3. Hire Website Designer

For "web design company" keyword --- google.com ranking falling 51 to 480
For "graphic design companies" keyword --- google.com ranking is 35 but currently it's not within 500
For "hire website" keyword --- google.com ranking is 21 but currently it's not within 500

I lost most of the visitors coming from google.com

I can't understand why my site is falling and what's the possible reasons.

Please help me.