My high school diploma is missing. I have no idea where I put it. Is there a way to get it reissued or certificated? ?

You should be able to contact the high school you graduated from and have them reissue it to you. You will probably have to pay for it, but they should have the ability to do that for you.

NEBOSH International General Certificate (IGC) provides participants with fundamental knowledge on workplace safety and health, trains them to detect, monitor and manage health risks and dangers, and enables them to apply the practical knowledge. The course has three units, and each one is assessed individually.

You contact your school or school district first. 

contact the school district that issued it

you might be able to get it from your high school if you contact them