That depends on how badly the brush was saturated with paint when it dried...
First off, if it is a cheap brush, just get rid of it. It is not worth the effort to fix it, compared to the cost of replacement.
If it is a good quality brush that was rinsed but not washed clean, you can try using "Motsenbocker's Lift Off # 5" (available at the big box stores and probably at all of your local paint dealers too) and a good brush comb and/or brass wire brush - but it will take time and effort, not to mention money if you do not have the Mostenbocker's or a brush comb... Usually restoring a brush in this condition is not worth the investment, especially if there was a decent amount of undiluted paint left over when the brush dried. (The more paint left in there, the more time it will take to clean.)
If the brush is a rock, toss it. No hope of restoring it to any kind of useful condition.