Welcome to the Community. I'd be glad to help you determine where your little report went.
The Budget vs Actuals report will generally be in the Business overview section of the Reports tab. I've included a picture to serve as a reference:
Just to clarify, you are using QuickBooks Online Plus, correct? The reason for asking this is because this report is only available in this version. For additional information on available reports in QuickBooks Online, please see the following link: Report Comparison Between QuickBooks Online Subscriptions.
If you are using Plus, I recommend verifying that you have a prepared budget in QuickBooks Online, as the report will be missing if there isn't one. You can check this by selecting the Gear icon and clicking Budgeting.
Lastly, if you do have prepared budgets and are still missing the Budget vs Actuals option in the Reports tab, I suggest seeing if you can run the report in the Budgets section. Here's how:
Select the Gear icon.
Select the drop-down arrow in the Action column and press Run Budgets vs Actuals report.
I've gone ahead and included a brief video to serve as a visual reference:
I'm confident that this information will help you get on track and locate the missing report.
Please feel free to let me know if I can be of additional assistance. Wishing you a wonderful start to 2019.