There is nothing like an Assembly not opening and a warning error message making an appearance to send a mild panic through you.
What to do? Is a question you see posted on the SOLIDWORKS Forum occasionally. Especially from those not on a current subscription!
I have seen this error a few times over the years. My experience with the message have occurred when we have been working on a reasonably large assembly on one computer and go to open it on another of our computers.
Once you get over the initial panic there is a simple procedure to work through the issue. Select the Assembly required to be Opened. Look for the Configuration box, select and change Default to Advance and then select Open. This will open a new window dialogue box – Configure DocumentSelect New configuration showing Assembly structure only. This will create a new configuration. Name the configuration something that is easily recognizable and select OK
I’ve never had a situation where a part or sub-assembly didn’t Unsuppress.
After I have unsuppressed all the parts & sub-assemblies I have been able to Saved the assembly. I would then open each of the parts & sub-assemblies from the Assembly, rebuild each and save. Once all appears to working correctly I delete the created configuration.
However if a part or sub-assembly didn’t unsuppress then at least, you hopefully, will have found the cause of the problem. If it is a sub-assembly then you can work through the same procedure until again, hopefully, you can isolate the problem to an individual part!
Hopefully you will never experience the Error message. But if you do, hopefully this will help you panic just a little less!