I had the same problem you did. I installed Gmail and Yahoo as email clients on my BB and went into problems, all kinds. I lost all my contacts. I spent days trying to "fix". researched this forum and don't know why no one steps up to help you.
Backup all your applications, data(emails, contacts, etc, and 3rd party (you can do it now) to the lastest Desktop Manager 6. Be sure to have your userid and password ready.
Then click on OPTIONS/Security Options/Security Wipe/check and click Emails, contacts, etc and nothing else. It will take a little while, and you might get a little nervous.
Then connect your BB to Desktop Manager and do a full restore to yur BB. Good luck!!
Warnings to you all. BB email is the best email available. Why overwrite it with Gmail and Yahoo where their defaults overcome BB's. I transfer my BB contacts to PC Outlook two ways - using Desktop Manager or if it does not work, I use ABC Converter. I don't want to talk too much here as I don't want to be responsible for your problems/and/or/data loss. Believe me, I even got into trouble using Gmail on PC to override Outlook and Comcast Emails. I lost all my Comcast emails, but luckily, I had a backup in the Outlook. Gmail is very powerful, but too powerful because it does assume the role of being the default email client and wipe yout data elsewehere.
Another point to make is that Outlook is just not 100% compatible with BB contacts and calendar. Mapping is supposed to be available, but it ia too complicated and insufficent. I can go on and on, but at least, now you know what to do if you get into the same mess BDunn, someone else on this forum, and I got. I am telling you not to get into this mess in the firsrt place.