blank admin page

I’m having a problem with a blog that I created for a client last year that I’m pretty sure was caused by an incompatible plugin, but removing the plugin did not fix the problem. The blog displays correctly and the login page is accessible and functional, but when I log in the admin page is blank. I’ve been on the forum looking for a solution and have tried a couple with no luck. I’ve removed all plugins via FTP. I’ve renamed the theme (both child and parent) in an attempt to force the default theme to display. This did not work so I changed the theme in the database and was able to get the blog to display the Twentyten theme—but still no admin page.

I would appreciate any ideas, but I assume the next step is to reinstall WordPress. I’m concerned about losing the client’s data. I think I can handle it but would feel more comfortable if I could confirm the reinstallation procedure. Can anyone point me to instructions on reinstallation that will allow me to retain existing data, theme, styles?

[No bumping, thank you.]