How can I get the synonyms back? Along with the disappearing red lines that only show up when you suspect a misspelling and right click on them, they don’t show up.
You will get spell checking and thesaurus only if you have dictionaries for them installed for the language that you are writing in. They are shown by a green tick in Character dialogue.
If the language for the text shown in the Status bar at the bottom of the screen is English (UK) or English (USA) then by default you should get spell check and thesaurus.
If the language is something like English (Zambia) or other language that doesn’t have a green tick or None then it won’t be spell-checked unless you have installed the relevant dictionaries (None never gets spell checked). Note that you could have spell check but not thesaurus.
If the text shows a non-standard language setting, such as En-US then it might have been written in a different word processor. Select the text and change the language to the expected variant, in this case English (USA).