Run-time error -2147352571 (80020005) Type Mismatch

[krissz]

I have suddenly started getting this error on the command "Main_Menu.Show" where Main_Menu is a form. This has been working perfectly till now. The only changes to the spreadsheet have been the addition of 3 new sheets, some new columns in one sheet, changes/additions to some equations plus the necessary extra subs to deal with this. There has been no changes to the form.
A web search revealed one suggestion, to do with Add-ins. Tried that but no help.
I have an original backup (which works) but am loath to abandon a day's work.

 

[krissz]

I have been experimenting on backups. With the only changes being to the data, this error keeps occuring.
Searching around, I gather that a solution may be to first define the UserForm. Any suggestions on how to do this, as just adding Dim Main_Menu as UserForm causes more error messages.