Directly from https://ellislab.com/expressionengine/user-guide/installation/upgrade_from_1.x.html :
Third-party add-ons from version 1.x will not work with ExpressionEngine 2.
Please contact each add-on’s author for a 2.x-compatible version before
upgrading, or your site may not function properly. The installer will
automatically disable all extensions as part of the upgrade process.
Big, and at the top of the page... you (whomever upgraded) definitely needed to vet which add-ons were in use and which ones had 2.x compatible versions before upgrading. I would highly recommend restoring backups to the original site and starting the process all over, but this time carefully examine all add-ons and find replacements for ones without compatible upgrades.
If there is no way to restore the site to previous 1.x functionality, all you can do is spend the time it will take figuring out what add-ons were in use and try to find suitable replacements quickly; the site will likely be broken until then.