It could be due to several distinct reasons.
1. If the report is a Historical report driven by dates, ensure you have filtered it right using the correct dates.
2. It could also be due to an applied filter in Reports, that is giving back 0 results.
This small icon in the top-left corner will help you identify the total number of rows in the report and if there is any applied filter.
To see it, click on the Info icon within the toolbar:
Then, place the mouse on the info icon (the left one in the screenshot above). This will show you the amount of filtered rows displayed in your report.
In parallel, you can also place the mouse on this Filter icon in the top-right corner to see if there’s any filter, avoiding data to be showed: