Error 500 Problem

After de Upgrade to 2.0 I I had problemes, never was the same. I just deleted the company, but is still there with no Clients, but if you go to create a lists of clients, the system tells you that the email is used, so all the information of the company is there but I can not see it, and I can not work.

Can you please tell me how can I delete all and start from zero ?

Kind regards,
Alejandro