[ilamont]

My Amazon seller account got deactivated 5 months ago with no specified reason.

Experienced Amazon Seller here. They always cite a reason for account termination, even if it's a ridiculous one, or one that's impossible to disprove ("we have reason to believe you are manipulating reviews of another account"). For non-fatal issues, Amazon often asks for a "Plan of Action" (POA) to have accounts reinstated.

You mentioned that you were out of action for 5 months. Did you check your notification history? Were you monitoring your seller metrics, FBM orders, feedback, and customer messages? If you don't respond to something like a customer message or a placed order that will definitely lead to problems up to and including account termination.

As for your specific issue - accessing tax records - seller forums indicate that you should still be able to log into the dashboard and access certain parts of the site. Can you do this? If so, have you attempted to file a support case for this issue?