I got up really early this morning and just hit a wall.

This is the reason someone said leaving the party.

Is hit a participle as in has hit or the past simple?

Hit the/a wall is an idiom. It means you just can go any further.

It's used in business or a project to mean you've met some kind of problem that will keep you from making further progress. In this case, it means the person is so tired they just can't go on - they have to go home and go to bed
simple past. (i persume)
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 BarbaraPA's reply was promoted to an answer.
Thank you all!

From the meaning, as GG says, the past simple 'hit' makes sense!