@Planet13 How old was that theme? For the past few years I've never had an issue with WP upgrades - even with themes that were heavily modified (Thesis, CoWorker, and Genesis). The basic rule of thumb for upgrades is still and should always be: a) make a backup copy of your database and b) all of your files as they are at the moment just before backup. Always.
It was an old theme 9magasin uno reloaded ver. 1.1). There was already an update out for the theme, but I DIDN'T update because I didn't know how to make child themes, and I didn't want to lose my hand-made modifications to the theme.
Yes, backups are a many splendid thing. I am glad I made mine, even though things worked out.