After opening the sessionStore.bak file I found that all the tabs were not being restore because they had been moved to the '_closedWindows' section. So I opened the file in an online json editor, copied the the 'tabs' section(including the header) under '_closedWindows' to 'Windows' where it should have been, deleted the original and everything is running smoothly again. Hope this helps someone else in the same desesperate situation.