I have had a few times when I've noticed my hormones going all crazy and libido basically disappearing (I've been on the pill for about 10 years now, and it's happened probably 3 times). Each time, what has helped me to get it back was to start having sex! I'd make myself do it, because it did still feel good and it was important to my partner. Eventually I realized that I was doing it for me again.
The second thing that has helped is someone new being interested in me (either just sexually or romantically). Inducing that "NRE"-ish kind of feeling tends to shock my system into overdrive. Hubby and I swing off and on, so when we've noticed a decreasing trend in either of our libidos we go have sex (or heavy flirting/petting) with someone new. Something about feeling desired is always a boost.