This all assumes the HPG network is responsible for the Fortress Republic Wall going up and is what maintains the Wall as well. That said .....
..... the Fortress Republic Wall goes up, but it causes a disaster to befall every world in known space. Unknown to anyone a computer virus is left behind in the HPG network by the old Comstar, all but forgotten about in the splitting of Comstar in 3052. The virus was meant to be a "backdoor" to allow Comstar to regain control of the HPG network, or individual HPG stations, if it was compromised by the houses of the Inner Sphere, or a Periphery nation. Any new HPG stations that have transmitted to received messages from "infected" HPG's received copies of the dormant virus as well. This virus unwittingly causes the Fortress Republic Wall to be established sphere wide on every world with a HPG. This causes each world with a HPG to become its own small no jump zone. While communications appeared to have failed, commerce continues. Unknown to anyone the jumpships that leave any star system with a HPG never arrive at their destination, but are destroyed. Any Jumpships arriving at any system with a HPG are destroyed upon arrival. In just a short period of less than two weeks, and before any way can be found to stop the jumpships from destroying themselves by jumping to or from worlds with HPG's, interstellar travel is almost completely destroyed. The Fortress Republic Wall even effects the Clans by traveling through their repeater HPG network to the Clan Homeworlds. The vast fleets of Jumpships, Dropships, Warships and all the trained crews manning these vessels are almost completely wiped out. By the time worlds realize what is happening, and disable or destroy the HPG stations causing the destruction, Humanity is in shambles. Outside of the Republic the massive fleets of jumpships are reduced from many thousands to a mere few hundred with the dropship fleets fairing only slightly better. No warships are know to have survived outside the Republic. The Republic stands alone as the only power ...... anywhere!
This is the new Battletech!