Well, first off, since you're fairly new to the Guild, I'd like to personally welcome you. So, welcome to the Guild! Certainly hope you're enjoying your time here and finding what you're looking for. And now to move onto your question with some personal input. 

Before I go on a mini-rant, I'd like to say that I've GMed and Co-GMed quite a lot over my five years here at RPG, but I wouldn't call myself "good" by any stretch of the imagination. This is because my Roleplays have the tendency to die, especially now that the site is becoming a bit more stagnant. However, I do think that if a GM is dedicated enough, it's possible to save a slow dying Roleplay. But this isn't only an effort on the GM's part, but also the players. 

If the players are no longer interested, then the GM will quickly find themselves in a bit of a tough spot. How does one handle this? Well, the GM should weed out any inactive or uninterested members and any members that can't post or are no longer interested should be upfront about it. I find that being upfront about losing interest tends to be pretty rare on RPG. But anyway, once the GM and players come to some sort of agreement of removing inactive members (or those who have lost interest), throwing up an interest check and recruiting new individuals would be a great way to get the gears going again.

There are other reasons a RP can die, but this tends to be the most common one. For some, there could be unrest in the OOC or the plot might feel clunky or awkward. For each situation, a GM and/or the players should come to a decision on how to fix the situation. It's not always possible to save a RP however and if the Roleplay ultimately meets its demise, don't give up! Try relaunching it with a different group, smaller group, in a different section, etc. It's amazing how much just one minor adjustment can change the entire dynamic of the Roleplay.