Microsoft 365 subscription vanished after I signed in with another account

It hasn't gone anywhere, it just lives on the other account. A 365 subscription belongs to the Microsoft account that paid for it, so Office only shows it while you're signed in as that account. In Word or Excel go to File > Account, sign out, then sign back in with the email you bought it with. Give it a minute and it should say Microsoft 365 again instead of asking you to activate. If you're not sure which email that was, search your inbox for the purchase receipt, or sign in to your Microsoft account online and look under Services & subscriptions on each one. Also watch out if the second account is a work or school one, Office prefers those when both are signed in, so remove it from the app if you don't need it there.