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It depends upon how you access your Facebook account usually... but, if you installed security software yesterday (MalwareBytes) that may have something to do with it.
It could be, that if you have a cookie set in your web browser, for your Facebook account, or if you access it by way of a saved address in the address bar (as part of your Address or URL History), MalwareBytes may have cleared that out.
The thing to do is, to search for and access your Facebook account again, if its cookie or history trace has been erased (or otherwise lost.)
Then there will be a cookie set, or a history trace, once more. And if that disappears again, you will need to set MalwareBytes to not erase such elements of your browser history or cookies.
But if I haven't covered the actual circumstances of your usually accessing your Facebook account adequately, let me know the actual circumstances, and I will advise you further and specifically.