Thank you for your reply. I just re-installed QCAD 3.4.6. It seems to work again.
I started to work on the QCAD 2 while I bought the pro version. I think that I saved Under a R15 dxf from the free version as after the re-instal, QCAD warned me about my file format. Then I saved it under R27.dxf format and everything sounds ok now. My files just need some quick changes but I feel better now as I didn't want to draw this for the 3rd time !
Thank you anyway for your time