This might be helpful.
Open After Effects and go to File > Import > Import a Premiere Pro project. If it imports fine , wait for it to load all the stuff and then go to File > Export > Export as a Premiere Pro project. Then try to open the project and see if it opens fine.
Note: You might lose some of your effects and transitions.
Hopefully you had a Project back up regime running and hopefully it was not relying on auto saves. Good luck.
This is a lesson to everyone.
Save Copy as ( "backup" - regularly)
Thanks for the replies. Rameez those suggestions were the first things Itried . Just to tidy up this thread here's how I solved the problem: luckily I was able to find some autosave files in the premiere pro documents folder. It seems that the scratch disk for the project had been changed so that auto saves went to my local disk instead of being beside the project. Don't know how it happened but thankful I didn't have to start the whole project from scratch again.