Jenny - I only know a little, myself. But when you change themes, you don't lose anything. The themes are like skins that fit over your basic information. Your widgets, settings, etc. are all preserved when you switch themes. So you don't have to be afraid of that.
Go into your dashboard, call up a post to edit. If the visual editor tab is selected - wait, are you saying there is NO visual editor tab, no HTML tab? Or are you just missing the editing options? Do you see text in the edit window? Does it have html characters in it, or is it just straight text?
I can't help you if the tabs aren't there. I've not run into that problem myself (thank heavens). But the smart people here will tell you: try another theme and see if that fixes it. If not, turn off your plug-ins, if you have any (I see that you're only running importer - but it may be causing something so try turning it off). But do these things one at a time, checking each time to see if there's a diff.
There is a very nice master-list for trouble-shooting that will teach you a lot.