iPad OS17.3.1 - Safari tabs

Without any description of precisely what you have attempted, contributors here have absolutely no insight into what you may (or may not) have found and tried when searching online.

Are you certain that you closed or lost all your open tabs - and have not, perhaps, opened a new Safari Window? Each Safari Window has its own self-contained set of open tabs.

When you open Safari, you may see the Shelf at the bottom of the screen - within which you will see each of your open Safari Windows; simply tap the thumbnail corresponding with the Safari Window (with associated tabs) that you wish to view.

If you closed a Safari Window - and all associated tabs - you can reopen a Recently Closed Window for a limited period of time…

Starting from a Home Screen, assuming that Safari is visible in the Dock, touch-and-hold (i.e., long touch) the Safari icon; an Action Menu will appear. 

From the menu, select Show All Windows; you will now see a screen that closely resembles the Task Switcher - but you will only see preview windows for your open Safari Windows. At top-right (or top-left for iPadOS16/17) you will see the Reopen Closed Window(s) button.

You should note that the Reopen Closed Window(s) button allows you to reopen a window for a limited period of time. If left too long, you may have lost the opportunity to recover the closed window - and with it the tabs and related tab history.

If you have accidentally closed all you your open tabs associated with a Safari Window, this User Tip may be helpful in recovering your tabs. While I have not [yet] updated the document to reflect changes in iPadOS15/16/17 (having been written for an earlier version of iPadOS), much of it is still relevant - as are the outlined strategies:

iPadOS - Safari - Lost Windows, Lost Tabs… - Apple Community

If your Safari tabs are being unexpectedly closed, check your Safari settings:

Settings > Safari > [Tabs] Close Tabs - select as preferred

Unless you select Manually, inactive open tabs will be automatically closed per the configured setting.

I have never attempted to quantify precisely how long that a closed Safari Window can be recovered - as this is largely determined by the iPad's memory and resource management. That said, the opportunity to recover/re-open a closed window is typically minutes/hours - not days or weeks.

Tapping an App icon within the Dock will, where supported, expose the associated shelf, this being an integral element of iPadOS multitasking. iPadOS permits you to have multiple App windows of the same App - each being self contained. The shelf, when triggered, shows a thumbnail of each Window associated with the App. Each App - such as Mail, Safari, Notes, Calendar, etc. - has its own corresponding shelf.

The Shelf is used to quickly switch between open windows or instances of the App. Individual windows can be closed directly from the Shelf; just swipe the unwanted thumbnail off the top of the screen.