Its best to throw it out. When you put the brush back in the tube to get more mascara. Don't pull brush in and out of the tube to get more. Your adding air to tube and this makes it dry out faster. Its best to put it in the tube and turn brush a couple of times. This will help your mascara to last longer and from drying out.
Toss It!! Mascara needs to be replaced every 3 months or so anyway... Even the slightest exposure to bacteria can produce an eye infection... Just not worth the 5 bucks to replace it...
You should probably get some new mascara soon so that you don't risk getting an infection. But until then try just putting 1 or 2 drops of water in the mascara bottle. Then use the brush end and work it around in the mascara it will loosen up whatever is left in it. I've done this many times you'll be surprised how good it works try it!
throw it away. if it's so old to be dried out, the preservatives aren't going to be any good so you run an increased risk of eye infections. just toss it.
I buy cheap mascaras, because that way if you get an eye infection or allergy to it it doesn't matter if you have to throw it out.
I used Maybelline, the one with the green lid and pink container. (I think it's Maybelline)