Starting super basic, so apologies if I go over something you already know:
- Mainsite FA and forum are separate accounts. Make sure that you have created accounts on both and are using the right password for your mainsite account.
- Try deleting any cookies you have that are associated with FA.
- If your account name includes underscores or capitalization, make sure you enter those in the username field, rather than your account name as it appears in your profile URL. I don’t know 100% if this is still true, but it used to be that the site could be rather picky about that.
- If you have access to more than one device (computer, phone, tablet, etc) and/or more than one Internet connection (home WiFi, cellular data, work WiFi, etc), try logging in using them to make sure it’s not your device/connection being wonky.
- If all else fails, try using the password recovery/reset feature. It’s possible (if unlikely) that something went awry and there’s something wrong with how your information is stored in the database.
- If password recovery also fails, try emailing accounts@furaffinity.net for assistance in recovering your account. Make sure you send your email from the same address as you have associated with your account. I can’t guarantee that this is still correct procedure for account recovery, so if @luffy/@Flamingo say otherwise please listen to them instead.

I’m pretty sure they meant “forum” and autocorrect did a number on it.