I haven't recently attempted an upgrade, but there was something of a SNAFU at my webhost (dreamhsot) and they have restored from a backup. Files were affected, MYSQL was not. I'm guessing what may have happened is the files are pre-upgrade, and the DB is post-upgrade - but I'm not certain, that's just a guess.
Each time I visit the gallery (www.hikingphotos.co.uk) it redirects to upgrade.php which gives:
No upgrade required, the database structure is up to date
Is there any way I can work around this?
what version of Piwigo did you have ?
2.4.0 ou 2.4.1 ? the last one is supposed to solved that problem : upload it by FTP