@Gargron
If you lose a password, you lose access to the data it protects. You can reset your account password, but you can't get access to the data again unless you remember the old password.

It is possible to export and import encryption keys for emails and contacts, which would allow you to back them up.

If you later remember your old password, after resetting it, you can get data back by reactivating the old keys with the old password:

https://protonmail.com/support/knowledge-base/restoring-encrypted-mailbox/