You can start with:
git reflog | grep 'to branchname'
This will match the reflog line as shown below:
219daf7 HEAD@{20}: checkout: moving from master to branchname
git checkout 219daf7, , , tig , tig 219daf7, . ( , git checkout , tig .)
, , , , :
git checkout -b branchname 219daf7
fsck:
git fsck --full --no-reflogs --unreachable --lost-found | grep commit
:
unreachable commit 219daf70a24e635cd95c1493c341585bbf64a61d
, .