[quote=@IAmTheIsland]
As a player, GMs ghosting is equally frustrating. Especially when they outright refuse to let another person step up and Co-GM. Had that happen once; GM hadn't been around. I asked about taking over in their stead, they denied, the RP ultimately died. Also once had a GM abandom her RP to take over another one and then, when asked for any notes/plot ideas she had had, her response was just "I don't care." 
[/quote]

This is indeed quite the conundrum. What I suggest doing next time this happens is just creating a new RP that closely resembles the old. Ideally you'll want to wait a few months to really make sure the old one is dead, but you can wait a year or longer if you wish. The important part is to make sure that there's a player base for the RP that's willing to move forward. If former players are eager to get back into their characters after months of not playing them, that's a good sign. Not everyone will want to come back, and that's fine. 

Discuss things with your player group a little bit. You want to understand why this was important to them and why they keep thinking about their characters. They will give you hints on how to run things, since you will more than likely take things in a different direction than the original GM would have. That is okay. The RP did not last with the original GM, and if your players have realistic expectations, they'll expect things to be different and be happy you can run things. 

The next step is to take what you know and just make a new RP. Depending on factors, you may choose to pick up right where the last RP left off, but I would insist on a time skip to help justify why certain characters aren't around and just give everyone an easier time finding their footing. No one's been RPing their characters for a bit so they may need a few post rounds to get back into character. And then the rest depends on you.