Hopefully new brushes will get your Dremel back in service, but here's what works for me:
I have an older version of the Dremel 4000 which has served me well for the past probably 10 years and is still running well.
About a year ago I got the Dremel 8050 Micro which is a light duty, rechargeable unit. I've ended up using it much more than my corded more powerful Dremel since it's on my bench, always charged, and ready to go. The built in LED light is actually pretty helpful for lighting up your work. It's pretty light weight so delicate work feels easier. If you use a flex shaft or any accessories that thread to the nose of the Dremel, the 8050 Micro isn't an option since it lacks the threads.