For preinstalled software (also called Original Equipment Manufacturer or OEM), the vendor -- Dell -- is responsible for all support. That's in return for Microsoft charging the vendor a greatly reduced price for the licenses.
Contact Dell's technical support group. If your computer is still under warranty, that's best. Have your service tag number and purchase paperwork handy. You may have to be
very insistent to get to a support representative who knows what to do.
The ideal solution is to get an Office installation disk so you can do a proper repair or reinstallation. Dell should make it available as a backup disk.