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 and HEAD, but keeps those which are different between the index and working tree (i.e. which have changes which have not been added).