Got trouble opening Lotus Notes

I got a Lotus Notes error message saying that my public key was not found in the Domino Directory. How can I fix this error? I have been using this successfully, but now, I could not even start it. What should I do? Do I need to reinstall my Lotus application? Please give me suggestions on how to fix this problem. Thank you. I am really confused with this.

IBM Lotus Notes
Server Error: Your public key was not found in the Domino Directory
