I have back ups but they are from like a month ago and are really old..those will open but anyway to update them?
Have you tried opening the assembly in a different mode than resolved? If the assembly has different configurations, can you change the configuration before opening, and will it open? Have you tried opening any sub assemblies or parts before opening the final assembly?
Usually that message means something is corrupt in the file and it will not open normally.
Try what Mark suggests and if that doens't work, you need to get in touch with your SOLIDWORKS reseller. They can help you and if they can't resolve it, they can escalate the issue to SOLIDWORKS Tech Support. This sounds like it might be a corrupted file which usually rquires us to fix.