Is Linode able to tell me my SSH/LISH credentials?

I'm sorry to hear that you're having trouble logging into your Linode! As Linode is primarily an infrastructure provider, Support does not have the ability to login to your Linode and view it's logs or perform any configurations/troubleshooting, nor do we have access to your Linode's credentials. You should be able to login to SSH/LISH using any username/password combination that you have created on the Linode.

If you have not created any users, or cannot remember, you can login as the 'root' user. You can access this account with the username 'root', and the password you created when you deployed your Linode. If you cannot remember this password, then you can reset it by following the instructions in our guide. Resetting your Linode's root password requires turning your Linode off briefly, but once you've gained access, you can reset any user's password by issuing the following command, where <username> is the name of the user who's password you are resetting:</username>

passwd <username>

If you haven't already, I highly recommend taking a run through our guide on Securing your Server, which discusses how to create a user, as well as how to properly secure SSH access using Public Key Authentication.

Just want to clarify one point, Tom is spot on if you're clicking the link in the Manager to get to Lish. But, if you're using a Lish gateway from the terminal, the first thing you need to do is login with your Manager username/password (or configure an SSH key). So if you have issues with that, you can request your username or reset your password via the normal website forms.