there could be several reason for that message.
What You can try is to rename the corresponding .cfg file and try to open it then again
I remember there were situations where the problem was only in the config file rather than in the assembly file itself.
Give it a try!
Do you work from on a network?
Because your file can become corrupt when you save and at same time something happens with your network.
We have had the same problem a couple of times, Siemens can repair it (Not always) but will take around 1-2 weeks.
depending on the nature of corruption... and the complexity of the assembly... it might be worth submitting to GTAC for review and recovery.
but, depending on the amount of work it would take to rebuild...
there is one another approach You can try.
Open a new assembly and try to put the old one into that.
If this is possible YOu can move the parts up to the new top assembly and save it again.
So You will get a repared ASM with all Your parts in it and with all relations as before
This sometimes works better than open it.