can git reset or checkout be reverted?

From: bill lam <cbill.lam@gmail.com>
Date: Feb 6, 2009, 14:19 UTC
Message-ID: <20090206141940.GB7231@b2j>

If I want to recall a old version of testing by

git reset --hard sha1 or git checkout sha1

then git log does not show anything beyond that commit. It does give some warning and recommend -b switch next time. If I only do that by accident or ignorance. How to revert to the original HEAD?

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