No sound or signal on playback

On several of my songs/projects, I am suddenly getting no sound on playback, and no levels are showing on the meters, even though I still see the wav files there. It was working fine until yesterday, and nothing has been changed since then.

I have checked my ASIO settings, my output settings, the tracks are not on mute, not on ‘monitor’ setting, all faders are up, sound interface is plugged in. Nothing is changed. The project looks the same as before, wav files are there, but there is no sound, and as the song plays/scrolls there are no levels showing, as if the signal info is simply not being read.

Step by step-
1 Create new project
2 record several audio and midi tracks
3 playback working fine
4 suddenly, no sound on playback and fader levels show nothing.

I have a PC, an Acer with Intel i7, 8gbRAM, Windows 8.1 64 bit, running Cubase LE AI Elements 8 64bit.

Please help. Thanks,