I created a spreadsheet with many rows but I can’t scroll up to the top.

Hard to tell what is going on without a screenshot but the most common problem is the table has been accidentally moved too far up on the sheet. It is off the top of the sheet so you cannot see the first few rows below the headers (but can see the headers).

Select the table. Grab the bullseye at the top left of the table. Drag downward until the table is fully on the sheet and all your rows are visible.

EDIT: That was advice for the Mac OS version of Numbers, which is the forum you posted in. I now see your signature is iOS. I do not use Numbers on iOS but it may still be the same solution.