The 180 Approach
If your motivation has evaporated along with your gains, it may be time to do something completely different, even if it takes you out of your comfort zone.
Sometimes the training system you normally use stops being beneficial. Not because it's flawed, but because you've adapted and it doesn't give you the same progressive results it used to.
Achieving mastery is like acing a test in subject. Once you do it, you can choose to go further and make this subject your all-encompassing focus, you can complement it with something else, or you can go master a different thing and change your goals.
The 180 approach will introduce something completely new. By turning 180 degrees, you'll have to open your eyes again. It can be frightening because it'll challenge your beliefs and comfort level. But being challenged is a good mark of progress and a clue that you're on the right path.
This doesn't mean going from barbells to dumbbells. That would be more like the same-but-different approach (which has its place too.) You'd have to go further than that and do something requiring different skills. For iron-lovers, this approach can mean things like yoga, running, cycling, martial arts, gymnastics, or a specific sport.
A Different Kind of Improvement
The benefit of moving differently will lead to improvement in some form: body comp, strength, performance, agility, mobility, stamina, etc. The key is to train in a way that supports your main goal. Going for a long distance run every day doesn't support a goal of carrying the most amount of muscle possible, but training to improve your aerobic system in a targeted way to boost recovery does.
Building explosive power through martial arts training can benefit your lifting and will probably increase your enthusiasm about training in general. It can add new meaning to movements and muscles. This doesn't mean you need to step into the ring or cage and be hit in the head. There's nothing saying you need to compete to reap the benefits.
Why stick to one training methodology when you can have the best of both (or several) worlds? The 180 approach gives you this opportunity. If you're stuck, you should embrace this kind of thinking as soon as possible to avoid following the same old path that takes you nowhere.