SOS! Deadline looming and new to Storyline - can't edit a slide I made, everything greyed out, three weeks left of free trial

As Judy said, you don't have any slides; you have a Master full of objects. I suspect you went to the Master and placed a background graphic on it. You may have tried several before you found one you liked. You were pleased with the result; so pleased in fact, that you forgot to close the Master view and return to the slide view before you started adding objects to the slide. Since you placed those objects on the "Master" layout, every new layout you create (like "Blank") will look like the master. Then when you return to Slide or Story view, every new slide you create will look like the "Blank" layout. That's the purpose and special power of master layouts. Since those objects are on a master, you cannot change any of them, except by changing them on the Master.

Choose View -> Slide Master. Select the top slide of the thumbnails on the left. If you roll over it, it will be named Master Slide "Graphite".  Press CTRL-A to select all. Everything will be selected. Press CTRL-X to cut all. Everything will be copied to the clipboard and deleted from the master. Choose View -> Slide View. You will see a blank slide. It will have a brown background, because that graphic is placed on the background of the master, and as such cannot be copied or cut. It will show on all new slides. Press CTRL-V to paste the clipboard. Everything will be copied to slide 1.1.