Recorded audio is not playing back

Hello - I’ve got what is probably an easily answered question, but I’ve looked around a fair bit and have yet to get an answer that works.
I am using Cubase LE 5 and have been able to use this in the past to record guitar tracks through a Lexicon Alpha. As of a few nights ago a project I was working on was working fine - tracks recorded as they should and I could playback and hear the output through the Lexicon Alpha’s headphone slot.
Now, from today when I try to play or record in Cubase no sound is recorded or plays back. I have not changed any VST settings, and I can still hear the sound of my guitar through the Lexicon Alpha (but not as playback of recorded tracks). When I try to record a track, the waveform does not appear as I go (just stays flat) and the track disappears as soon as I press stop.
When I try to listen to any projects I have already recorded, the waveforms appear as usual, but when I press play no sound appears to even register in the program (the levels don’t move at all as the marker goes over the waveform). Does anyone have any idea why this might be?
My VST settings are as follows:
vst and cubase.png

Why is your output mono? Is everything routed to it?

Originally I was using stereo, then at some point it stopped working. The combination of settings you saw seemed to be the only way that I could get both the mic to record in cubase, and the output to play. I have a feeling that the problem is not related to this however (though this is admittedly another separate problem), as the audio that I am trying to play doesn’t seem to register at all (as in it doesn’t even attempt to output any music - also as the input doesn’t register anything when I record. If you look in the mixer, there is no movement on any of the volume bars etc. I’ve tried unmuting, switching the monitor on and off and solutions suggested in other threads, but doesn’t seem to work.

Let’s start at square one. Follow the set-up steps in this link. Make sure to go step by step.

http://www.steinberg.net/forum/viewtopic.php?f=25&t=10051

Exactly what are the mic and speakers connected to, same device?