Since installing high sierra the thunderbolt ports are not functioning, how do I fix this?

Darn that's annoying. Zap pram and reset the SMC/PMU (how to do that can be found on Apple's support page for your general type of Mac (notebook or Desktop Mac) or if you know the model number/type (found in About this Mac, under the Big Apple icon at the top left of your screen) you can do the Google or Yahoo or whatever to find out

should fix it----dunno why, but it usually does

John B