I'ts a personal account, but I joined a small business account ...
When you join a Business account without creating a new account, you merge your current account into the Business account, effectively making the business the owner of the account. At this point it's only your account in that you're a member of their team, using one of their licenses.
Why can't I log in? Why did this happen? How can someone just simply destroy my personal account just because I don't work for them anymore?
Assuming the above is true and you were a member of a Business account, when they decide to remove you from the team, the account you're using is deleted and is no longer accessible. If their intent is to return the account back to you, then they should have converted the account back to an individual account instead.
Once an account has been removed the team admin has seven days to restore it. After it's been restored, they can then convert it back to an individual account for you.
And I can't make a new account with my personal email address ... Is it possible to deactivate the account so I can make a new one?
If you can't get the team admin to restore and convert your account for you, then you'll be able to re-use your email address to create a new account once the original account has been purged after the seven days.