If your project is still active in the old version, then a member of the tdadmin group can undo the check out of the entity. However when a check oout is reverted, the entity is returned to the state it was prior to the check out and all changes are lost.
as was previously mentioned , the tdadmin user can undo the checkout, but be warned that the changes that were created during that check out will be lost and the entity will be reverted to the previous state before the check out.
However, if you are using QC Authentication (and not LDAP) and need to keep the changes, then you can change the password for the user within the Site Admin>Site Users tab and then log in as that user and ''check in'' the entity to preserve the changes. This only works with QC Authentication.
If you are in the process of upgrade when you discover this and the project is unavailable in the prior version (version already uninstalled, server down, etc that make accessing the project an impossibility, then you will have to clear the VC check out tables of the checked in entities (these can be found from within the project expanded in Site Admin or within the project tables in the DB) in the site admin of the upgraded version. After this is done then the project can be reverified, repaired, and upgraded as normal.
Hope this helps,