No problem. Bear in mind that they don't want to have a bunch of tours close to games, so try to book something as early in the week as possible, or they may tell you, "No dice."
Even if you can't get a guided tour, you can still see many things. The practice fields are pretty much open to the public during the daytime, although you shouldn't go on them. The "Self-Guided Tour" is neat if you just want to see our trophies (including all five National Championship Trophies) and some of our Husker honors, for most sports. You can walk into the stadium itself from 8am-8pm all week long at Gate 24, although you don't get to go on the field. You can walk out on the balcony overlooking the SE corner of the end zone, though, and that will give you a great look at the whole stadium.
Also, bear in mind that the stadium is under construction right now, so it's a bit of a mess on the East side.