Hey imorgs,
Welcome to the Apple Support Communities! From what you mentioned, you're having a bit of an issue with tabs in Safari. I'd be glad to see what I can do to assist.
There is a specific setting on a Mac for this that can be found in Safari > Preferences > General. For that, you'd want to make sure that "All windows from last session" is checked. If it is and you still don't get the previous tabs, uncheck and then recheck the setting.
Also make sure that there is plenty of space available on your device. Info on checking your Mac storage can be found below.
How to free up storage space on your Mac
Have a great day!