Silence can be hard to deal with, but it is the most common response you'll get from roleplayers. Most people don't have the back bone to tell you they aren't interested anymore. The problem is compounded because they could also be going through a family emergency or other crisis that prevents them from coming online. 

First check their activity. From there I would do the following.

If they have been offline for two weeks, they may still be interested. Do start the RP, Do make sure they can join at a later date. They might become a valuable asset to you later on However...

If they have been online semi-daily or are ghosting, move on without them. Absolutely do not wait another day for them. The role players you currently have ready will start to lose interest the longer this drags on.