Error message when opening an essembly

Graham - have you tried to open the copies of the assembly file from the Auto Recovery folder or from one on the copies stored in File Backups folder?

Hmmm, I'm afraid you have to do the assembly all over again, it's useless now.
Are the parts okay?

They seem to be and all the sub assemblies open ok, its just the main assembly

These days I had the same problem and I was very worried.
On the answer Tunc Gerdan, fixed the problem and for
that, many, many, many thanks

Open the assembly first and then try to open the drawing...

If that doesn't work then open SW RX and open with the last option

Same issue happened to me after changing the default appearance, I accidentally changed it to an appearance that I deleted from its original location. My problem was fixed by doing the following;

- Open Solidworks as normal
- Before you open any parts, assemblies or drawings just go to the appearance manager located on the right panel that has an icon with colourful ball.
- From the Appearance Manager, right click on any default appearance and select set as default appearance.
-Restart Solidworks and it should be working.

I hope this helps someone.

Good luck!