Another Leopard/Parallels upgrade 4 problem

I have installed Leopard on my Macbook and downloaded parallels 4. At first all was well then suddenly I click on the parallels icon and Parallels opens, I them click on resume, the screen turns and then freezes with the resuming bar almost to the end. I cannot get out of this and have to restart my Macbook

Ant suggestions, detailed info please, not very bright.