Practice what initially inspired you to pick up the guitar in the first place. The idea with this is the reason why you picked up the guitar in the first place must have been a strong enough reason to be motivated to practice. Or try going to other way with it. I find in similar moments that sometimes its cool to learn a style or genre you've never played before. It can feel a breath of fresh air.
Also, is there a reason you are aware of that has put out the fire? I sometimes feel unmotivated for the following;
- I haven't slept enough (in which case I need more early nights)
- I haven't eaten enough (in which case I should just eat more, yay!)
- I'm practicing songs that take more time to perfect (in which case, I need to find more short term goals)
- I play the same thing over and over, causing frustration (in which case I need to avoid repetition
- I don't practice enough, and I blame myself for it (in which case I need to organise my time)
Ed's right though. Completely agree. I am completely knackered from doing nothing, and have probably knocked my sleeping pattern about 6 hours ahead (or behind, I'm not quite sure these days!) Just know, and I mean JUST KNOW, this is all temporary. I've started practicing and listening to bands that inspired me again, such as BFMV, Black Stone Cherry, Sum 41 and also even given in to some cheesy pop stuff for easy listening. Keep music varied and hopefully something you either haven't listened to before or something you haven't listened to in years will push the motivation.