1. You start Minecraft
2. Load a world save
3. Click the Open to LAN option
4. Start Minecraft on his computer
5. Click multiplayer
6. Add server and put in right IP-address of the server
7. Select the server on list and connect
Those are the steps to accomplish what you want.
Did you install any new firewall or antivirus software that could block connection?
I guess MC defaults to server in the background and others can join in same network (bit weird tbh), must have changed or you are doing something different.
Try getting the IP-address of your computer by openin command prompt and typing "ipconfig /all" and it should show you it (something like 192.168.11.xx).
Write it down and then start your client and load world, the on friends computer click multiplayer and add server. Give it name and put the IP-address there, save and connect to it by doubleclicking the name or choosing it and click connect (it also should get green gauge next to name indicating your server is ok).