Multiplayer not working after version downgrade

When I went to version 1.0 of Minecraft and then upgraded back to 1.21, the multiplayer servers would not work. I tried to report this to the server I tried to join, Hypixel, but I hadn't made an account on Hypixel's website yet, and the bug reports required an account link from Minecraft. However I couldn't join the server to use the verification link command, so I went to Mojang's official site. Can someone tell me how to fix this?