Amata LireinRan into the same problem just now. Write the other player a PM, then give him/her a reasonable time to respond (lets say a week), afterwards either just abandon the arc or, if other players are involved as well try to figure out how to continue either way without godmodding.
I fully understand that I've taken the proper steps towards informing her, and even so, that was a week ago. I'm going to inform her again, and a simple edit or another post could be more than enough to make a comprehensive story that would enable the arc to continue at a later date. Thankfully it's only with one other person, and a bit of creativity is all that's needed. So thank you for the help! I really mean it