Soft brick after dunking phone in coffee
My brother's S3 Mini had a nice water trip on Espain beach. It turned on, but it had software issues such as apps opening randomly, volume changing itself, and random reboots. When the battery went empty, it didn't turn on. 

My brother tried his friend's S3 Mini battery, and it worked fine, but all the problems remained. So, he bought a new one.

Try another battery. If still doesn't boot, take it to the service center. Last thing is to buy a new phone.