It's not easy. There are a few ways to get re-motivated but then those don't always work.
You can start training for a different race/distance. If you're doing 5ks look at doing a solid marathon, or vise versa; drop down from marathon training and tackle the short fast stuff.
Find a race that you always wanted to do and plan on traveling to it and really trying to nail it.
Start running with a group, even if it's just once a week. You won't want to fall behind, so you'll get in your miles the rest of the time.
And then finally, sometimes you need to step away from running for awhile. Take 6 mo. and just hike, go to the gym, ride a bike, rock climb whatever...