Hi,
Expanding on the two previous suggestions since you may not know where the default workbook is kept or how to modify it:
The default workbook, which is usually named Book.xlt is generally located in the C:\Program Files\Microsoft Office\Office 11\XLStart folder. (note Office 11 is just one possibility depending on the version of Excel you are using.) You can always do a window's search for XLStart.
Open the file in Excel using File, Open. Modify the file as necessary - 1. Unhide Sheet1 if its hidden, 2. Add Sheet1 if it has been deleted.
There is one other possible cause of your problem - there could be a Workbook_Open macro that either hides or deletes sheet1 every time you open the file. This could be located in the Book.xlt file, the Personal Macro Workbook, or some add-in.
Cheers,
Shane Devenshire
Microsoft Excel MVP