I had this situation many times and established some sort of "catalogue" for me, what could be done against this:
1. Oblique Strategies - always a good first choice.
2. As a guitarist, try different tunings, especially ones, which you never have tried before.
3. Listen to some of your old stuff, which you never have continued, because at that time you didn´t had any ideas for it - maybe now´s the time.
4. Establish a "Composing Hour", which means that every day at the same time (that´s best, though different times of the day also work), you sit with all your equipment (guitars, keyboards, computer, software, boxes - all that you have) and force yourself to DO something for one hour - then stop and don´t think about the music until you sit there again the next day. When you have a new idea, just record a short sketch of it. Keep away from starting arrangements, into deep sound tweaking etc. - keep it fast and simple and move on to sth. else. There might be little to none output at the beginning, but the continuity of everyday´s working rhythm will get you on the track again. After one month you listen to your recorded material and then start to add more arrangements, sounds etc. (I got this idea from a very good book on this subject: "The Art & Craft of Writing Music" by Matthew Nicholl, http://www.lulu.com/product/paperback/art-and-craft-of-writing-music/16071216 , which was originally recommended by Robert Fripp)
5. Read a book. Try subjects that are not your favourite crime reading etc., but sth. new to you.
6. Go to the cinema. Try films that are less action and more to contemplate about afterwards.
7. Go to a concert. Especially with music, that is not your favourite band, but somebody you just have read about in the papers or sb. had told you it´s interesting.
8. Go to a museum or exhibition. If you have talent in doing sketches, take some paper with you and try to make a sketch of one piece of art, which you liked very much. Put this sketch on the wall where you work on your music. Let it inspire you, watch it while you play your instrument, let your musical thoughts meander into every possible direction and do a simple recording of what you play
9. Leave all your musical stuff completely untouched for at least one week (or two, or three...) and DON´T worry about it.
10. Okay....buy a box ;-). This one looks interesting, too: http://www.youtube.com/watch?v=j7-f-t28GOM&feature=BFa&list=PLA548FE27426F8D8C&lf=mh_lolz
Hope this helps a bit :-)