lost access to our company youtube channel after an employee left

Check one thing before chasing the ex-employee: did they sign in with their work email, a Google Workspace address on your own domain? If so you may not need them at all. Your Workspace admin can reset that account's password in the admin console, log in as it, and add you as an owner on the channel. Quickest fix by far when it applies. It only works while the account still exists though, so tell whoever handles IT not to delete leavers' accounts until channels and logins have been moved over.

Depends how the channel was set up. If it's on a Brand Account, check whether anyone still at the company is listed as an owner or manager. If someone is, they can add you, and an owner can later be made primary owner (there's a waiting period of about a week as an owner before that's allowed). If the channel lives on the ex-employee's personal Google account, you need them. Ask them, politely and in writing, to add a company address as owner and then transfer primary ownership. Most people just do it if asked early. If they won't or can't be reached, go through YouTube's account recovery and support with proof the channel belongs to the business: trademark, business registration, invoices that mention the channel, the website it links to. It's slow. Afterwards, keep it on a Brand Account owned by a company email nobody leaves with, and give staff manager access only.