You can try tap-and-holding the + at the bottom of the screen showing the open tabs. It will present you with a list of the recently closed tabs. You might be able to find the sites you are looking for.

This list can also be seen in the history section (with the clock icon) of the bookmarks.

For the future I suggest you save the sites in your favorites where it's less likely you'll loose them. When seeing a web page click on the share button (box with an upwards arrow) at the bottom of the screen. Scroll the menu to find Add to favorites.

Afterwards you can find the favorites by tapping on the bookmarks icon (a book).