I went throught the very same thing a couple years ago.And lets behonest, it is bound to happen if you spend a long time in the hobby. One of my tanks got old tank syndrom and was on the verge of a pH crash. What has worked for me was getting my head in some books. For some reason, I hate most reading but even when in a slump I still enjoy reading about my aquatic hobby.
Spend some time touring LFS in your area, try going to one that you have never been to before and see their fish, tanks, and ask questions. The kind of questions that you have always wanted to know but never knew the answers. Sure some stores might not be able to answer you nine times out of ten, but this way you are starting the thought proccess that will lead you to investigation.
Also another thing that you can try is to do something that you have never done before with your tank. For example, if you have never tried live plnats get a couple of those. Never had co2? Put in a DIY co2 system. Try making your tank look like a certain biotope, or a location in the world. Always wanted to feed more live food? Start your own brime Shrimp hatchery and feed them to your fish. All of these things are pretty affordable(although, some more than others) and can rejuvenate your passion fot the hobby.
If all these things seem to fail, take a year or two away from the hobby. Sell your fish (but keep your tank!) and let it sit for a year or two. Once you "discover" your tank in the basement one day, you may find that your passion for fish keeping instantaniously jumps back.