It looks like you are on lemmy.world. It's possible it could have been because of the update listed below to that instance, I actually had to sign out and back in cause I couldn't upvote, subscribe etc:

_The lemmy.world instance was just updated to version 0.18.2. The login issues that were being reported (for example, here) are now resolved.

For release notes: https://lemmy.world/post/1339018_

Edit for those who still have issues logging in:

• When using a browser: clear cookies and cache
• When using an app: remove your lemmy.world account and add it again.

That's the full post above^ Still learning how to comment on Lemmy since I was mostly a lurker on Reddit 😂