Safari tabs suddenly disappearing on my iPad

Assuming that the Safari icon is in your Dock at the bottom of the screen, swipe upwards from the bottom edge of the screen to the centre - and pause briefly before lifting your finger from the screen. This will open the App Switcher.

While the App Switcher is open, touch-and-hold (i.e., long touch) the Safari icon in the Dock; an action menu will appear. Tap Show All Windows option; the Task Switcher will now only show the currently open Safari instances/windows. At top-right or top-left of this view, you should see an option to Re-open recently closed Window(s). Tap this button to reopen your closed Safari instances - complete with open tabs and associated tab-histories.

Alternatively, from a Home Screen, touch-and-hold (i.e., long touch) the Safari icon; an action menu will appear as before. Tap Show All Windows - and Reopen Closed Windows at top-right of the screen.

Be aware that the opportunity to recover closed Safari Windows is time limited - and will not survive a restart of the iPad.

You may find this article to be helpful in understanding some of the frequent difficulties encountered by users new to iPadOS Safari - along with some of Safari’s more esoteric features and quirks:

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

Insofar as possible - and while written for earlier versions of iPadOS - I hope to have distilled most of the frequent problems reported here - and documented procedures, strategies [and where needed] some useful workarounds.

I that you find this to be helpful in resolving any issues that you may be experiencing.