I get that sometimes too. I want to draw stuff but I'm not inspired to do anything more than conceptualize.
Don't do art then Stop thinking about doing and do something else. You'll be hit by the creativity bug soon. Read books, watch tv, watch movies, bike around, listen to new music, etc. OR get some R&R in a far away place that isn't your city or your house or your room, haha (like go hiking, take a walk, go to the mall). I find that these help to shake things up when I'm feeling stagnant.
Maybe you should actively look for stuff to do, like maybe start accepting commissions and offering to take up requests and such. Join contests and do things with a deadline!
Groups are fine and all but find a group that requires you to submit a minimum number of pieces (per week/per month, whatever ) for you to stay a member, haha. I'm part of DAC (Draw a Character group here on dA) but I've been lazy for the past 6 months and not joining in the prompts that they have. I'm WAAAAY behind now, actually, haha.
I find that sometimes, added pressure is necessary to get my butt to work, haha. (the healthy kind, mind you Not the pressure that will overwork you and spread you out too thin).
I find for me creativity comes in cycles. I will have so many ideas I can not possibly do them all.....and then I have no ideas. Used to be when I had no ideas, I worked on technical skills and problem areas. If you are needing work on perspective, do some 1, 2 and 3 point perspective pieces, if you tuck hands behind the back, and end all drawings mid-leg to avoid feet and hand, get pictures of legs and picutres of hands and work on those. Now, I have a book of ideas. When I have so many ideas I can not possibly do them all, I make notes about the ones I am passing up. I carry this notebook everywhere, so when I am at work, or the grocery store and an idea strikes I can note it for later. You could also pick up one of the 100 things challenges that is floating around DA, do it as a 1 drawing a day challenge as well, and it will not even take you 4 months to get it all done.
Maybe you're a little bored and need to force yourself to do something completely different. You could try sitting down and doing some still life to the best of your ability. Or go to the library and check out all the books you can on drawing and then actually make yourself do some of the practices in them. For me doing something that seems technical and tedious often helps to loosen the creative block and makes it easier to go back to drawing fun thing. It's also a good way to develop skills that you can apply to your more creative work. You really have to focus at it though and make yourself do the work... even if it gets boring after 10 minutes and you feel like playing on the internet or watching TV you have to stay focused.
Try going to Illustration Friday. Every week they select a new prompt and you have until next Friday to finish it. It's Saturday now so you have 6 days to work on the current topic. Or pick one of the previous ones. Whatever you like.
You don't need other people telling you what to draw and I'm perfectly okay with saying that after you're just stated you want other people to tell you what to draw.
Relaxe, your sketchbook is for random junk, practice and trying things out. It's okay if its shit you don't want anyone else to see because they don't have to! So draw whatever you want! A page of butts. Tropical birds. Skeletons playing various sports. Whatever you can think of.
Ideas are like lego bricks you put together in your brain. When you say you lack ideas I don't believe you.