Welcome to the forum. I'll try to answer your question. If you have a previously saved assembly, you should be able to open it and any features belonging to the assembly should be unchanged (planes, mates, sketches, etc.). Parts referenced by this assembly are a different story. If the parts used in this assembly have been edited, then those changes will show up.
It's for this reason that when I have design changes I frequently use the Pack and Go feature. That copies an assembly (or drawing), along with all referenced files, to a new folder. You can then make changes to the assembly and referenced parts with much less chance of making unwanted changes to previous versions. You can also re-name components as part of the Pack and Go process, which I don't always do, but it will pretty much eliminate any chance of referencing the wrong documents.
if you have the back-ups set up in the system options, you may be able to get back the version you are looking for. otherwise this is what a PDM system is for. in either case you would have to retrieve the previous versions of the parts and assembly separately (unles they are virtual parts, then they are seved in the assembly file and would come with it).