John: Martha, how do I fix the whatchamacallit on the computer? The internet thingie?
Martha: You mean the router? Just unplug it and plug it back in.

“Whatchamacallit” is a word we can use when we don’t remember the proper name of something. If a person knows what you are trying to do, they might know what you mean. But usually you will have to give more details to help people guess what you are talking about.