Hello,
I am sorry to hear of these problems. You could open an SFTP connection to your site, take a backup of your existing .htaccess file and try creating a new one with the default WordPress version here.
You may also wish to try temporarily renaming the wp-content/plugins folder to something else to rule out a plugin conflict.
We also have some instructions here for manually restoring a site that you cannot access which may be of use. If you do not wish to create a new installation then you could use method 2.
Best,
Paul