These "network path was not found" types of problems are generally related to third party security suites. Either the installation or update / upgrade of such products can often times result in this issue, so...
I would start there. If you are using or have in the recent past used products such as Norton / Symantec, McAfee, AVG, etc., uninstall them using the control panel, programs and features applet and then Google for the vendor specific, proprietary removal tool and get rid of any and all remnants of said product.
Additionally it may sometimes be linked to credential corruption so check the credential manager on both machines and get rid of any credentials on either machine pointing to the other and attempt to connect again at which point you should be prompted for credentials again. Enter the MachineName followed by a backslash "\", then the username (the machine name you are attempting to access and a user name on that machine with proper permissions to access the shares you are trying to get to) and that user's password on that machine. Do not use usernames without passwords, you need to use properly configured usernames and passwords.
Lastly make sure both machines are members of the same "Work Group", and that there are no duplicate machine names or IP address on the network.
Make sure you can ping one machine from the other using the IP address as well as the NetBIOS name of each machine if either or both of these fail then you likely have an issue with a third party security suite and I would suggest again that you get rid of them and go back to using Microsoft Security Essentials and the native windows Firewall product on both until such time as you can resolve this issue.
Norton Removal tool =
https://support.norton.com/sp/en/us/home/current/solutions/kb20080710133834EN_EndUserProfile_en_us AVG Removal tool =
http://www.avg.com/us-en/utilities McAfee Removal tool =
https://service.mcafee.com/FAQDocument.aspx?id=TS101331 You can find other using Google.
EDIT: Sorry forgot to also mention to double check the dates and times settings on both computers and assure that they are the same.