|Quoting legs (Reply 1):|
The first thing that comes to mind is a wing sweep actuator failure, though there's probably a couple of other things like control circuitry or feedback mechanism failures that will stop you from sweeping the wings.
I know the F-111 flight manual had charts for landing speeds at different wing sweeps, and according to some of the pilots back in the day it wasn't a huge issue to land with the wings swept back a bit. Obviously, its going to be faster and depending on how swept the wings are the spoilers may well be locked out so you'll use a lot of runway and work the brakes pretty hard.
As far as taking off, they'd probably use way too much runway for it to ever used normally, maybe a ferry flight with only minimal fuel would be the only way to even get off the ground.