Hi, keycuts, & welcome to the WordPress support forum. There are an overabundance of reasons for this sort of behavior.
The first thing that would *really* be helpful to us in terms of trying to diagnose the problem would be for you to provide a site url. It can tell us a great deal.
Have you tried logging in using wp-login.php as opposed to wp-admin, i.e. https://www.yoursite.com/wp-login.php? Does that work?
If you’re using a theme not bundled with WordPress (those themes begin w/the name twenty), then the next thing we generally recommend is to switch to one of these. Since we’re assuming now that you cannot log in, then you can do this by renaming the folder of your currently active theme by putting a 1 or a .old at the end of the folder name. Note: This *WILL* break the homepage, but if it allows you to log in, then you can change your theme, & your site will be restored, albeit somewhat altered in appearance.
If the above steps don’t fix the problem, then we suggest renaming your plugins folder, again by putting a 1 or a .old at the end of your plugins folder. So it looks like plugins1, for example. If you can now log in, then that indicates a conflict w/1 of your plugins. Rename the folder back to the original name, but append a 1 to the names of all folders inside your plugins folder except for one. Probably the most likely candidate to leave alone is akismet, since that’s a bundled plugin developed by the WordPress core developers & is therefore unlikely to contain incompatibilities. Then try logging into your site again. Keep renaming plugins back to their original names, 1 at a time, clearing the browser cache, & then logging in each time until you find the plugin that prevents you from doing so.
Please let us know how it all goes.