Black video after exporting sequence in FCP7 - help please!

So i'm not doing anything new - i've edited my clips as usual, settings are all as usual, but after i export the finished sequence and try to play it back, all i get is a black screen, with audio, for the entire duration of the sequence.
in the canvas it plays back fine, theres nothing to render, i've tried both with H.264 and Apple prores 422 HQ and nothing, i have tried to export as QT and also using QT conversion, checked the video settings there, all good, the video is rendered, also i checked the video processing in settings, used both render in RGB and render in 8 bit YUV, but no luck.
and i dont think it has to do with in and out points because the black video I get IS the duration of the audio, and the whole sequence for the matter.
The only thing I am wondering about is that I added some png files to make "gifs" over the video - I exported this same video BEFORE adding the pngs and everything was fine, but it is after adding these still images that the whole video goes black after exporting. But I have done similar projects before, with the same type of files, and I have never gotten this problem.
note; there are only a few of these still images scattered over the sequence, the majority of the video doesnt have these pngs over, but it is the whole of the video that goes black after exporting.
Can someone help me please, i dont know what else to give a go at and i cant find answers to my problem online!
many thanks.


Could be your sequence is corrupt.  Create a new sequence with the same settings and copy everything from the old sequence to the new sequence. 
If you need help with this post back.