I use the Tor Browser to access the internet, but when I log into my Facebook account it has been locked and shows a security message from Facebook informing me that someone has tried to log into my account from Portugal. I am living in Sri Lanka.
What would be the issue? Did someone try to hack my Facebook account via Tor Browser?
Facebook incorrectly recorded your logins. This seems unlikely; I am not aware of any reports of this.
Facebook detected one of your logins as coming from Portugal, despite you not being in Portugal. This was addressed by cacahuatl’s answer: the most likely answer is that you logged in via a Tor circuit that exited in Portugal. To follow up on the comments on that answer: Tor Browser tries to make all users look the same in every respect, not just disguise their IP address, so other information, such as the user agent, may also appear incorrect.
Facebook detected (correctly or incorrectly) someone else’s login as coming from Portugal. This leads to the second part of your question, discussed below.
Did someone try to hack my Facebook account via Tor Browser?
Based on your question, it is possible that someone tried to “hack” your Facebook account, but there is no reason to suppose that they used Tor Browser to do so. But as I write this, you have already accepted cacahuatl’s answer; hopefully that means answer 2 above was the correct one.