How to Reactivate Dried Acrylic Paint
To reactivate acrylic paint once it has dried try cellulose car paint thinner, it does a pretty good job at dissolving/thinning dried up acrylic paint.
Cellulose thinner has powerful and toxic vapours and is highly inflammable. It's widely used within the auto-spraying industry and you need a suitable respirator.
If you do use it, make sure you use it outdoors or in a very well-ventilated area, keep away from heat sources and above all, read and heed the instructions/warnings on the tin!