Can't Login Anymore!

Sorry if I post here, but I cannot write in the "troubleshooting" forum...

Recently my provider changed the permissions to 'index.php' due to a mail spam attack.
I changed back the permissions to 644 but the login as user and/or administrator doesn't works anymore!
No errors, no warnings: simply the page refresh after sending username and password and nothing changes...
Anything else in the site is still working but the chart, becouse no order is accepted without login.
Also password reset is not working: I receive the mail to reset the password, but it links to the usual login page...
I tried changing other permissions, but nothing changes!
Version 4.1.3

Any suggestion?

Please share or PM me URL of your website. The issue cannot be linked with index.php permissions. It is common script for all store-front pages and it cannot work just for several pages

I'm going to guess that there were changes in either the SSL certificate on your site or the "secure" settings in your store.

Please verify both. If you swtich from a NON ssl page to an SSL page (I.e. login) and then back again, the session could be getting reset. Verify that you have the "maintain https" (may be named differently) checkbox checked.