With git reflog check which commit is one prior the merge (git reflog will be a better option than git log). Then you can reset it using:
git reset --hard commit_sha
There’s also another way:
git reset --hard HEAD~1
It will get you back 1 commit.
Be aware that any modified and uncommitted/unstashed files will be reset to their unmodified state. To keep them either stash changes away or see --merge option below.
As @Velmont suggested below in his answer, in this direct case using:
git reset --hard ORIG_HEAD
might yield better results, as it should preserve your changes. ORIG_HEAD will point to a commit directly before merge has occurred, so you don’t have to hunt for it yourself.
A further tip is to use the --merge switch instead of --hard since it doesn’t reset files unnecessarily:
git reset --merge ORIG_HEAD
–merge
Resets the index and updates the files in the working tree that are different between <commit> and HEAD, but keeps those which are different between the index and working tree (i.e. which have changes which have not been added).