KeighleyHistory on "Internal server error"

[Moderator note: Topic title de-capped. Please do not shout at us.]

I was attempting to activate Plug-in "Spreadsheet" last night and got this

Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator to inform of the time the error occurred and of anything you might have done that may have caused the error.

More information about this error may be available in the server error log.

The server error log shows

WordPress database error Table 'wordpress_9.wp_data' doesn't exist for query SELECT * FROM wp_data made by require('C:\Inetpub\vhosts\keighleyhistory.org.uk\httpdocs\wp-blog-header.php'), require_once('C:\Inetpub\vhosts\keighleyhistory.org.uk\httpdocs\wp-load.php'), require_once('C:\Inetpub\vhosts\keighleyhistory.org.uk\httpdocs\wp-config.php'), require_once('C:\Inetpub\vhosts\keighleyhistory.org.uk\httpdocs\wp-settings.php'), include_once('C:\Inetpub\vhosts\keighleyhistory.org.uk\httpdocs\wp-content\plugins\dhtmlxspreadsheet\spreadsheet.php'), sh_load_dump WordPress database error Duplicate entry 'demo_sheet' for key 'PRIMARY' for query INSERT INTOwp_dhx_sheetVALUES ('demo_sheet', null, null, 'any_key', null) made by require('C:\Inetpub\vhosts\keighleyhistory.org.uk\httpdocs\wp-blog-header.php'), require_once('C:\Inetpub\vhosts\keighleyhistory.org.uk\httpdocs\wp-load.php'), require_once('C:\Inetpub\vhosts\keighleyhistory.org.uk\httpdocs\wp-config.php'), require_once('C:\Inetpub\vhosts\keighleyhistory.org.uk\httpdocs\wp-settings.php'), include_once('C:\Inetpub\vhosts\keighleyhistory.org.uk\httpdocs\wp-content\plugins\dhtmlxspreadsheet\spreadsheet.php'), sh_load_dump

Now I can't even log in!!!!

Anybody got any ideas?