You can do any of following:
1- Change the engine in the first phase, if you have planned to do it later, do it now. Save money on existing engine repair.
2- If engine is not rotating, you will have to open engine head to see what inside. Since you drained mix of oil and water, there is strong possibility that head gasket is shot, and pistons are stuck, may be due to water presence, or in worst case, engine has been hydro locked (may be the car engine sucked in water during rain). Opening head will reveal its further treatment.