Unprotecting a spreadsheet

I am working with a large spreadsheet that at one point I must have protected in some way, as I am not allowed to delete four of the worksheets. I don't remember what exactly I did, as it was quite some time ago. The sheets really do need to be deleted.

Is there a limit to the number of sheets that you can have for a single Excel page? I don't know if its related to my problem with not being able to delete the worksheets or not, but when I go to the Insert (at the top) and look to add a worksheet, it is not available as an option (grey instead of black).

Any advice?

Cheers,

Dee