Although a repair may not be permanent, you can repair either a silicone or urethane rubber mold so that you can at least get a few more castings. You can then use that a casting to make a new mold.

Repairing a silicone mold – use Sil-Poxy silicone rubber adhesive. Important; all surfaces to be bonded must be clean. Use acetone or other solvent to clean the area to be bonded before applying adhesive.

Repairing a urethane mold 
– use Ure-Bond urethane rubber adhesive. Important; all surfaces to be bonded must be clean. Use acetone or other solvent to clean the area to be bonded before applying adhesive.